diff --git a/Cargo.lock b/Cargo.lock
index 2e7cc018923..bdd5cc0ceee 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6088,6 +6088,7 @@ dependencies = [
  "canvas_traits",
  "crossbeam-channel",
  "euclid",
+ "fnv",
  "fonts",
  "fonts_traits",
  "html5ever",
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index e7713b11674..f85f6c676e4 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -1865,6 +1865,7 @@ where
                     pipeline.title = title;
                 }
             },
+            FromScriptMsg::IFrameSizes(iframe_sizes) => self.handle_iframe_size_msg(iframe_sizes),
         }
     }
 
@@ -2138,11 +2139,6 @@ where
     fn handle_request_from_layout(&mut self, message: FromLayoutMsg) {
         trace_layout_msg!(message, "{message:?}");
         match message {
-            // Layout sends new sizes for all subframes. This needs to be reflected by all
-            // frame trees in the navigation context containing the subframe.
-            FromLayoutMsg::IFrameSizes(iframe_sizes) => {
-                self.handle_iframe_size_msg(iframe_sizes);
-            },
             FromLayoutMsg::PendingPaintMetric(pipeline_id, epoch) => {
                 self.handle_pending_paint_metric(pipeline_id, epoch);
             },
diff --git a/components/constellation/tracing.rs b/components/constellation/tracing.rs
index 06ee3ee44e3..c81da81fa89 100644
--- a/components/constellation/tracing.rs
+++ b/components/constellation/tracing.rs
@@ -189,6 +189,7 @@ mod from_script {
                 #[cfg(feature = "webgpu")]
                 Self::GetWebGPUChan(..) => target!("GetWebGPUChan"),
                 Self::TitleChanged(..) => target!("TitleChanged"),
+                Self::IFrameSizes(..) => target!("IFrameSizes"),
             }
         }
     }
@@ -257,7 +258,6 @@ mod from_layout {
     impl LogTarget for script_traits::LayoutMsg {
         fn log_target(&self) -> &'static str {
             match self {
-                Self::IFrameSizes(..) => target!("IFrameSizes"),
                 Self::PendingPaintMetric(..) => target!("PendingPaintMetric"),
             }
         }
diff --git a/components/layout/display_list/builder.rs b/components/layout/display_list/builder.rs
index bece8e09557..4046f45f314 100644
--- a/components/layout/display_list/builder.rs
+++ b/components/layout/display_list/builder.rs
@@ -13,7 +13,7 @@ use std::sync::Arc;
 use std::{f32, mem};
 
 use app_units::{Au, AU_PER_PX};
-use base::id::{BrowsingContextId, PipelineId};
+use base::id::PipelineId;
 use bitflags::bitflags;
 use canvas_traits::canvas::{CanvasMsg, FromLayoutMsg};
 use embedder_traits::Cursor;
@@ -25,7 +25,9 @@ use ipc_channel::ipc;
 use log::{debug, warn};
 use net_traits::image_cache::UsePlaceholder;
 use range::Range;
-use script_layout_interface::{combine_id_with_fragment_type, FragmentType};
+use script_layout_interface::{
+    combine_id_with_fragment_type, FragmentType, IFrameSize, IFrameSizes,
+};
 use servo_config::opts;
 use servo_geometry::{self, MaxRect};
 use style::color::AbsoluteColor;
@@ -43,7 +45,7 @@ use style::values::computed::{ClipRectOrAuto, Gradient};
 use style::values::generics::background::BackgroundSize;
 use style::values::generics::image::PaintWorklet;
 use style::values::specified::ui::CursorKind;
-use style_traits::{CSSPixel, ToCss};
+use style_traits::ToCss;
 use webrender_api::units::{LayoutRect, LayoutTransform, LayoutVector2D};
 use webrender_api::{
     self, BorderDetails, BorderRadius, BorderSide, BoxShadowClipMode, ColorF, ColorU,
@@ -327,7 +329,7 @@ pub struct DisplayListBuildState<'a> {
 
     /// Vector containing iframe sizes, used to inform the constellation about
     /// new iframe sizes
-    pub iframe_sizes: FnvHashMap<BrowsingContextId, euclid::Size2D<f32, CSSPixel>>,
+    pub iframe_sizes: IFrameSizes,
 
     /// Stores text runs to answer text queries used to place a cursor inside text.
     pub indexable_text: IndexableText,
@@ -1872,19 +1874,25 @@ impl Fragment {
             },
             SpecificFragmentInfo::Iframe(ref fragment_info) => {
                 if !stacking_relative_content_box.is_empty() {
-                    let browsing_context_id = match fragment_info.browsing_context_id {
-                        Some(browsing_context_id) => browsing_context_id,
-                        None => return warn!("No browsing context id for iframe."),
+                    let Some(browsing_context_id) = fragment_info.browsing_context_id else {
+                        return warn!("No browsing context id for iframe.");
+                    };
+                    let Some(pipeline_id) = fragment_info.pipeline_id else {
+                        return warn!("No pipeline id for iframe.");
                     };
 
                     let base = create_base_display_item(state);
                     let bounds = stacking_relative_content_box.to_layout();
 
-                    // XXXjdm: This sleight-of-hand to convert LayoutRect -> Size2D<CSSPixel>
-                    //         looks bogus.
                     state.iframe_sizes.insert(
                         browsing_context_id,
-                        euclid::Size2D::new(bounds.size().width, bounds.size().height),
+                        IFrameSize {
+                            browsing_context_id,
+                            pipeline_id,
+                            // XXXjdm: This sleight-of-hand to convert LayoutRect -> Size2D<CSSPixel>
+                            //         looks bogus.
+                            size: euclid::Size2D::new(bounds.size().width, bounds.size().height),
+                        },
                     );
 
                     let pipeline_id = match fragment_info.pipeline_id {
diff --git a/components/layout_2020/context.rs b/components/layout_2020/context.rs
index f1542327422..871fce6cb0d 100644
--- a/components/layout_2020/context.rs
+++ b/components/layout_2020/context.rs
@@ -11,7 +11,7 @@ use net_traits::image_cache::{
     ImageCache, ImageCacheResult, ImageOrMetadataAvailable, UsePlaceholder,
 };
 use parking_lot::{Mutex, RwLock};
-use script_layout_interface::{PendingImage, PendingImageState};
+use script_layout_interface::{IFrameSizes, PendingImage, PendingImageState};
 use servo_url::{ImmutableOrigin, ServoUrl};
 use style::context::SharedStyleContext;
 use style::dom::OpaqueNode;
@@ -35,6 +35,9 @@ pub struct LayoutContext<'a> {
     /// A list of in-progress image loads to be shared with the script thread.
     pub pending_images: Mutex<Vec<PendingImage>>,
 
+    /// A collection of `<iframe>` sizes to send back to script.
+    pub iframe_sizes: Mutex<IFrameSizes>,
+
     pub webrender_image_cache:
         Arc<RwLock<FnvHashMap<(ServoUrl, UsePlaceholder), WebRenderImageInfo>>>,
 }
diff --git a/components/layout_2020/display_list/mod.rs b/components/layout_2020/display_list/mod.rs
index 0d848141874..2be4d005e30 100644
--- a/components/layout_2020/display_list/mod.rs
+++ b/components/layout_2020/display_list/mod.rs
@@ -6,11 +6,9 @@ use std::cell::{OnceCell, RefCell};
 use std::sync::Arc;
 
 use app_units::Au;
-use base::id::BrowsingContextId;
 use base::WebRenderEpochToU16;
 use embedder_traits::Cursor;
 use euclid::{Point2D, SideOffsets2D, Size2D, UnknownUnit};
-use fnv::FnvHashMap;
 use fonts::GlyphStore;
 use gradient::WebRenderGradient;
 use net_traits::image_cache::UsePlaceholder;
@@ -32,7 +30,6 @@ use style::values::generics::NonNegative;
 use style::values::specified::text::TextDecorationLine;
 use style::values::specified::ui::CursorKind;
 use style::Zero;
-use style_traits::CSSPixel;
 use webrender_api::units::{DevicePixel, LayoutPixel, LayoutRect, LayoutSize};
 use webrender_api::{
     self as wr, units, BorderDetails, BoxShadowClipMode, ClipChainId, CommonItemProperties,
@@ -162,12 +159,6 @@ pub(crate) struct DisplayListBuilder<'a> {
     /// The [DisplayList] used to collect display list items and metadata.
     pub display_list: &'a mut DisplayList,
 
-    /// A recording of the sizes of iframes encountered when building this
-    /// display list. This information is forwarded to layout for the
-    /// iframe so that its layout knows how large the initial containing block /
-    /// viewport is.
-    iframe_sizes: FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>,
-
     /// Contentful paint i.e. whether the display list contains items of type
     /// text, image, non-white canvas or SVG). Used by metrics.
     /// See <https://w3c.github.io/paint-timing/#first-contentful-paint>.
@@ -175,12 +166,13 @@ pub(crate) struct DisplayListBuilder<'a> {
 }
 
 impl DisplayList {
+    /// Build the display list, returning true if it was contentful.
     pub fn build(
         &mut self,
         context: &LayoutContext,
         fragment_tree: &FragmentTree,
         root_stacking_context: &StackingContext,
-    ) -> (FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>, bool) {
+    ) -> bool {
         #[cfg(feature = "tracing")]
         let _span = tracing::trace_span!("display_list::build", servo_profiling = true).entered();
         let mut builder = DisplayListBuilder {
@@ -191,10 +183,9 @@ impl DisplayList {
             is_contentful: false,
             context,
             display_list: self,
-            iframe_sizes: FnvHashMap::default(),
         };
         fragment_tree.build_display_list(&mut builder, root_stacking_context);
-        (builder.iframe_sizes, builder.is_contentful)
+        builder.is_contentful
     }
 }
 
@@ -318,11 +309,6 @@ impl Fragment {
                     builder.is_contentful = true;
                     let rect = iframe.rect.translate(containing_block.origin.to_vector());
 
-                    builder.iframe_sizes.insert(
-                        iframe.browsing_context_id,
-                        Size2D::new(rect.size.width.to_f32_px(), rect.size.height.to_f32_px()),
-                    );
-
                     let common = builder.common_properties(rect.to_webrender(), &iframe.style);
                     builder.wr().push_iframe(
                         rect.to_webrender(),
diff --git a/components/layout_2020/flexbox/layout.rs b/components/layout_2020/flexbox/layout.rs
index 9e705e5e5d3..668477cbd32 100644
--- a/components/layout_2020/flexbox/layout.rs
+++ b/components/layout_2020/flexbox/layout.rs
@@ -1987,8 +1987,11 @@ impl FlexItem<'_> {
                     }
                 }
 
-                let fragments = replaced
-                    .make_fragments(item_style, size.to_physical_size(container_writing_mode));
+                let fragments = replaced.make_fragments(
+                    flex_context.layout_context,
+                    item_style,
+                    size.to_physical_size(container_writing_mode),
+                );
 
                 Some(FlexItemLayoutResult {
                     hypothetical_cross_size,
diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index e3a45b579b6..47deeb10767 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -1399,6 +1399,7 @@ impl ReplacedContents {
     fn layout_in_flow_block_level(
         &self,
         base: &LayoutBoxBase,
+        layout_context: &LayoutContext,
         containing_block: &ContainingBlock,
         mut sequential_layout_state: Option<&mut SequentialLayoutState>,
     ) -> BoxFragment {
@@ -1420,7 +1421,7 @@ impl ReplacedContents {
 
         let containing_block_writing_mode = containing_block.style.writing_mode;
         let physical_content_size = content_size.to_physical_size(containing_block_writing_mode);
-        let fragments = self.make_fragments(&base.style, physical_content_size);
+        let fragments = self.make_fragments(layout_context, &base.style, physical_content_size);
 
         let clearance;
         if let Some(ref mut sequential_layout_state) = sequential_layout_state {
@@ -2071,7 +2072,12 @@ impl IndependentFormattingContext {
                     sequential_layout_state,
                 ),
             IndependentFormattingContextContents::Replaced(contents) => contents
-                .layout_in_flow_block_level(&self.base, containing_block, sequential_layout_state),
+                .layout_in_flow_block_level(
+                    &self.base,
+                    layout_context,
+                    containing_block,
+                    sequential_layout_state,
+                ),
         }
     }
     pub(crate) fn layout_float_or_atomic_inline(
@@ -2099,7 +2105,7 @@ impl IndependentFormattingContext {
                         &content_box_sizes_and_pbm,
                     )
                     .to_physical_size(container_writing_mode);
-                let fragments = replaced.make_fragments(style, content_size);
+                let fragments = replaced.make_fragments(layout_context, style, content_size);
 
                 let content_rect = PhysicalRect::new(PhysicalPoint::zero(), content_size);
                 (fragments, content_rect, None)
diff --git a/components/layout_2020/positioned.rs b/components/layout_2020/positioned.rs
index fe193878203..ecb65af97cc 100644
--- a/components/layout_2020/positioned.rs
+++ b/components/layout_2020/positioned.rs
@@ -572,6 +572,7 @@ impl HoistedAbsolutelyPositionedBox {
                         block: block_axis.size.to_definite().unwrap(),
                     };
                     fragments = replaced.make_fragments(
+                        layout_context,
                         &style,
                         content_size.to_physical_size(containing_block_writing_mode),
                     );
diff --git a/components/layout_2020/replaced.rs b/components/layout_2020/replaced.rs
index 149b5fe119b..86345e0b8f5 100644
--- a/components/layout_2020/replaced.rs
+++ b/components/layout_2020/replaced.rs
@@ -10,9 +10,11 @@ use app_units::Au;
 use base::id::{BrowsingContextId, PipelineId};
 use canvas_traits::canvas::{CanvasId, CanvasMsg, FromLayoutMsg};
 use data_url::DataUrl;
+use euclid::Size2D;
 use ipc_channel::ipc::{self, IpcSender};
 use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
 use pixels::Image;
+use script_layout_interface::IFrameSize;
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
 use style::computed_values::object_fit::T as ObjectFit;
@@ -280,6 +282,7 @@ impl ReplacedContents {
 
     pub fn make_fragments(
         &self,
+        layout_context: &LayoutContext,
         style: &ServoArc<ComputedValues>,
         size: PhysicalSize<Au>,
     ) -> Vec<Fragment> {
@@ -349,6 +352,15 @@ impl ReplacedContents {
                 image_key: video.as_ref().map(|video| video.image_key),
             })],
             ReplacedContentKind::IFrame(iframe) => {
+                let size = Size2D::new(rect.size.width.to_f32_px(), rect.size.height.to_f32_px());
+                layout_context.iframe_sizes.lock().insert(
+                    iframe.browsing_context_id,
+                    IFrameSize {
+                        browsing_context_id: iframe.browsing_context_id,
+                        pipeline_id: iframe.pipeline_id,
+                        size,
+                    },
+                );
                 vec![Fragment::IFrame(IFrameFragment {
                     base: self.base_fragment_info.into(),
                     style: style.clone(),
diff --git a/components/layout_2020/taffy/layout.rs b/components/layout_2020/taffy/layout.rs
index 5d21914278c..4ed13aa008b 100644
--- a/components/layout_2020/taffy/layout.rs
+++ b/components/layout_2020/taffy/layout.rs
@@ -174,8 +174,11 @@ impl taffy::LayoutPartialTree for TaffyContainerContext<'_> {
                         // Create fragments if the RunMode if PerformLayout
                         // If the RunMode is ComputeSize then only the returned size will be used
                         if inputs.run_mode == RunMode::PerformLayout {
-                            child.child_fragments =
-                                replaced.make_fragments(style, content_box_size);
+                            child.child_fragments = replaced.make_fragments(
+                                self.layout_context,
+                                style,
+                                content_box_size,
+                            );
                         }
 
                         let computed_size = taffy::Size {
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index f897b93d888..a01ae8c424d 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -15,7 +15,7 @@ use std::sync::{Arc, LazyLock, Mutex};
 
 use app_units::Au;
 use base::cross_process_instant::CrossProcessInstant;
-use base::id::{BrowsingContextId, PipelineId};
+use base::id::PipelineId;
 use base::Epoch;
 use embedder_traits::resources::{self, Resource};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect, Size2D as UntypedSize2D};
@@ -47,7 +47,7 @@ use layout::traversal::{
 };
 use layout::wrapper::ThreadSafeLayoutNodeHelpers;
 use layout::{layout_debug, layout_debug_scope, parallel, sequential};
-use log::{debug, error, trace, warn};
+use log::{debug, error, trace};
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use metrics::{PaintTimeMetrics, ProfilerMetadataFactory};
 use net_traits::image_cache::{ImageCache, UsePlaceholder};
@@ -61,12 +61,12 @@ use profile_traits::{path, time_profile};
 use script::layout_dom::{ServoLayoutElement, ServoLayoutNode};
 use script_layout_interface::wrapper_traits::LayoutNode;
 use script_layout_interface::{
-    Layout, LayoutConfig, LayoutFactory, NodesFromPointQueryType, OffsetParentResponse, Reflow,
-    ReflowGoal, ReflowRequest, ReflowResult, TrustedNodeAddress,
+    IFrameSizes, Layout, LayoutConfig, LayoutFactory, NodesFromPointQueryType,
+    OffsetParentResponse, Reflow, ReflowGoal, ReflowRequest, ReflowResult, TrustedNodeAddress,
 };
 use script_traits::{
-    ConstellationControlMsg, DrawAPaintImageResult, IFrameSizeMsg, LayoutMsg as ConstellationMsg,
-    PaintWorkletError, Painter, ScrollState, UntrustedNodeAddress, WindowSizeData, WindowSizeType,
+    ConstellationControlMsg, DrawAPaintImageResult, PaintWorkletError, Painter, ScrollState,
+    UntrustedNodeAddress, WindowSizeData,
 };
 use servo_arc::Arc as ServoArc;
 use servo_atoms::Atom;
@@ -130,9 +130,6 @@ pub struct LayoutThread {
     /// Is the current reflow of an iframe, as opposed to a root window?
     is_iframe: bool,
 
-    /// The channel on which messages can be sent to the constellation.
-    constellation_chan: IpcSender<ConstellationMsg>,
-
     /// The channel on which messages can be sent to the script thread.
     script_chan: IpcSender<ConstellationControlMsg>,
 
@@ -186,7 +183,7 @@ pub struct LayoutThread {
     paint_time_metrics: PaintTimeMetrics,
 
     /// The sizes of all iframes encountered during the last layout operation.
-    last_iframe_sizes: RefCell<FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>>,
+    last_iframe_sizes: RefCell<IFrameSizes>,
 
     /// Debug options, copied from configuration to this `LayoutThread` in order
     /// to avoid having to constantly access the thread-safe global options.
@@ -204,7 +201,6 @@ impl LayoutFactory for LayoutFactoryImpl {
             config.id,
             config.url,
             config.is_iframe,
-            config.constellation_chan,
             config.script_chan,
             config.image_cache,
             config.resource_threads,
@@ -306,16 +302,6 @@ impl Layout for LayoutThread {
         get_the_text_steps(node, &self.indexable_text.borrow())
     }
 
-    fn query_inner_window_dimension(
-        &self,
-        browsing_context_id: BrowsingContextId,
-    ) -> Option<Size2D<f32, CSSPixel>> {
-        self.last_iframe_sizes
-            .borrow()
-            .get(&browsing_context_id)
-            .cloned()
-    }
-
     fn query_nodes_from_point(
         &self,
         point: UntypedPoint2D<f32>,
@@ -529,7 +515,6 @@ impl LayoutThread {
         id: PipelineId,
         url: ServoUrl,
         is_iframe: bool,
-        constellation_chan: IpcSender<ConstellationMsg>,
         script_chan: IpcSender<ConstellationControlMsg>,
         image_cache: Arc<dyn ImageCache>,
         resource_threads: ResourceThreads,
@@ -571,7 +556,6 @@ impl LayoutThread {
             url,
             is_iframe,
             script_chan,
-            constellation_chan,
             time_profiler_chan,
             registered_painters: RegisteredPaintersImpl(Default::default()),
             image_cache,
@@ -755,48 +739,6 @@ impl LayoutThread {
         );
     }
 
-    /// Update the recorded iframe sizes of the contents of layout and when these sizes changes,
-    /// send a message to the constellation informing it of the new sizes.
-    fn update_iframe_sizes(
-        &self,
-        new_iframe_sizes: FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>,
-    ) {
-        let old_iframe_sizes =
-            std::mem::replace(&mut *self.last_iframe_sizes.borrow_mut(), new_iframe_sizes);
-
-        if self.last_iframe_sizes.borrow().is_empty() {
-            return;
-        }
-
-        let size_messages: Vec<_> = self
-            .last_iframe_sizes
-            .borrow()
-            .iter()
-            .filter_map(|(browsing_context_id, size)| {
-                match old_iframe_sizes.get(browsing_context_id) {
-                    Some(old_size) if old_size != size => Some(IFrameSizeMsg {
-                        browsing_context_id: *browsing_context_id,
-                        size: *size,
-                        type_: WindowSizeType::Resize,
-                    }),
-                    None => Some(IFrameSizeMsg {
-                        browsing_context_id: *browsing_context_id,
-                        size: *size,
-                        type_: WindowSizeType::Initial,
-                    }),
-                    _ => None,
-                }
-            })
-            .collect();
-
-        if !size_messages.is_empty() {
-            let msg = ConstellationMsg::IFrameSizes(size_messages);
-            if let Err(e) = self.constellation_chan.send(msg) {
-                warn!("Layout resize to constellation failed ({}).", e);
-            }
-        }
-    }
-
     /// Computes the stacking-relative positions of all flows and, if the painting is dirty and the
     /// reflow type need it, builds the display list.
     fn compute_abs_pos_and_build_display_list(
@@ -850,8 +792,8 @@ impl LayoutThread {
                     build_state.root_stacking_context.overflow = origin;
 
                     // We will not use build_state.iframe_sizes again, so it's safe to move it.
-                    let iframe_sizes = std::mem::take(&mut build_state.iframe_sizes);
-                    self.update_iframe_sizes(iframe_sizes);
+                    *self.last_iframe_sizes.borrow_mut() =
+                        std::mem::take(&mut build_state.iframe_sizes);
 
                     *self.indexable_text.borrow_mut() =
                         std::mem::take(&mut build_state.indexable_text);
@@ -1130,7 +1072,10 @@ impl LayoutThread {
         }
 
         let pending_images = std::mem::take(&mut *layout_context.pending_images.lock().unwrap());
-        Some(ReflowResult { pending_images })
+        Some(ReflowResult {
+            pending_images,
+            iframe_sizes: self.last_iframe_sizes.borrow().clone(),
+        })
     }
 
     fn update_scroll_node_state(&self, state: &ScrollState) {
diff --git a/components/layout_thread_2020/lib.rs b/components/layout_thread_2020/lib.rs
index 6dc5a204556..0010f68e022 100644
--- a/components/layout_thread_2020/lib.rs
+++ b/components/layout_thread_2020/lib.rs
@@ -16,7 +16,7 @@ use std::sync::{Arc, LazyLock};
 
 use app_units::Au;
 use base::cross_process_instant::CrossProcessInstant;
-use base::id::{BrowsingContextId, PipelineId};
+use base::id::PipelineId;
 use base::Epoch;
 use embedder_traits::resources::{self, Resource};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect, Size2D as UntypedSize2D};
@@ -35,7 +35,7 @@ use layout::query::{
 };
 use layout::traversal::RecalcStyle;
 use layout::{layout_debug, BoxTree, FragmentTree};
-use log::{debug, error, warn};
+use log::{debug, error};
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use metrics::{PaintTimeMetrics, ProfilerMetadataFactory};
 use net_traits::image_cache::{ImageCache, UsePlaceholder};
@@ -52,8 +52,8 @@ use script_layout_interface::{
     ReflowRequest, ReflowResult, TrustedNodeAddress,
 };
 use script_traits::{
-    ConstellationControlMsg, DrawAPaintImageResult, IFrameSizeMsg, LayoutMsg as ConstellationMsg,
-    PaintWorkletError, Painter, ScrollState, UntrustedNodeAddress, WindowSizeData, WindowSizeType,
+    ConstellationControlMsg, DrawAPaintImageResult, PaintWorkletError, Painter, ScrollState,
+    UntrustedNodeAddress, WindowSizeData,
 };
 use servo_arc::Arc as ServoArc;
 use servo_atoms::Atom;
@@ -115,9 +115,6 @@ pub struct LayoutThread {
     /// Is the current reflow of an iframe, as opposed to a root window?
     is_iframe: bool,
 
-    /// The channel on which messages can be sent to the constellation.
-    constellation_chan: IpcSender<ConstellationMsg>,
-
     /// The channel on which messages can be sent to the script thread.
     script_chan: IpcSender<ConstellationControlMsg>,
 
@@ -164,9 +161,6 @@ pub struct LayoutThread {
     /// Paint time metrics.
     paint_time_metrics: PaintTimeMetrics,
 
-    /// The sizes of all iframes encountered during the last layout operation.
-    last_iframe_sizes: RefCell<FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>>,
-
     /// Debug options, copied from configuration to this `LayoutThread` in order
     /// to avoid having to constantly access the thread-safe global options.
     debug: DebugOptions,
@@ -180,7 +174,6 @@ impl LayoutFactory for LayoutFactoryImpl {
             config.id,
             config.url,
             config.is_iframe,
-            config.constellation_chan,
             config.script_chan,
             config.image_cache,
             config.resource_threads,
@@ -295,15 +288,6 @@ impl Layout for LayoutThread {
         get_the_text_steps(node)
     }
 
-    fn query_inner_window_dimension(
-        &self,
-        _context: BrowsingContextId,
-    ) -> Option<Size2D<f32, CSSPixel>> {
-        // TODO(jdm): port the iframe sizing code from layout2013's display
-        //            builder in order to support query iframe sizing.
-        None
-    }
-
     #[cfg_attr(
         feature = "tracing",
         tracing::instrument(skip_all, fields(servo_profiling = true), level = "trace")
@@ -505,7 +489,6 @@ impl LayoutThread {
         id: PipelineId,
         url: ServoUrl,
         is_iframe: bool,
-        constellation_chan: IpcSender<ConstellationMsg>,
         script_chan: IpcSender<ConstellationControlMsg>,
         image_cache: Arc<dyn ImageCache>,
         resource_threads: ResourceThreads,
@@ -548,7 +531,6 @@ impl LayoutThread {
             id,
             url,
             is_iframe,
-            constellation_chan,
             script_chan: script_chan.clone(),
             time_profiler_chan,
             registered_painters: RegisteredPaintersImpl(Default::default()),
@@ -569,7 +551,6 @@ impl LayoutThread {
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
             webrender_image_cache: Default::default(),
             paint_time_metrics,
-            last_iframe_sizes: Default::default(),
             debug: opts::get().debug.clone(),
         }
     }
@@ -623,7 +604,8 @@ impl LayoutThread {
             image_cache: self.image_cache.clone(),
             font_context: self.font_context.clone(),
             webrender_image_cache: self.webrender_image_cache.clone(),
-            pending_images: Mutex::new(vec![]),
+            pending_images: Mutex::default(),
+            iframe_sizes: Mutex::default(),
             use_rayon,
         }
     }
@@ -867,7 +849,11 @@ impl LayoutThread {
         }
 
         let pending_images = std::mem::take(&mut *layout_context.pending_images.lock());
-        Some(ReflowResult { pending_images })
+        let iframe_sizes = std::mem::take(&mut *layout_context.iframe_sizes.lock());
+        Some(ReflowResult {
+            pending_images,
+            iframe_sizes,
+        })
     }
 
     fn update_scroll_node_state(&self, state: &ScrollState) {
@@ -935,8 +921,7 @@ impl LayoutThread {
             display_list.build_stacking_context_tree(&fragment_tree, &self.debug);
 
         // Build the rest of the display list which inclues all of the WebRender primitives.
-        let (iframe_sizes, is_contentful) =
-            display_list.build(context, &fragment_tree, &root_stacking_context);
+        let is_contentful = display_list.build(context, &fragment_tree, &root_stacking_context);
 
         if self.debug.dump_flow_tree {
             fragment_tree.print();
@@ -963,8 +948,6 @@ impl LayoutThread {
                 .remove_unused_font_resources(keys, instance_keys)
         }
 
-        self.update_iframe_sizes(iframe_sizes);
-
         if self.debug.trace_layout {
             layout_debug::end_trace(self.generation.get());
         }
@@ -1009,48 +992,6 @@ impl LayoutThread {
         }
     }
 
-    /// Update the recorded iframe sizes of the contents of layout and when these sizes changes,
-    /// send a message to the constellation informing it of the new sizes.
-    fn update_iframe_sizes(
-        &self,
-        new_iframe_sizes: FnvHashMap<BrowsingContextId, Size2D<f32, CSSPixel>>,
-    ) {
-        let old_iframe_sizes =
-            std::mem::replace(&mut *self.last_iframe_sizes.borrow_mut(), new_iframe_sizes);
-
-        if self.last_iframe_sizes.borrow().is_empty() {
-            return;
-        }
-
-        let size_messages: Vec<_> = self
-            .last_iframe_sizes
-            .borrow()
-            .iter()
-            .filter_map(|(browsing_context_id, size)| {
-                match old_iframe_sizes.get(browsing_context_id) {
-                    Some(old_size) if old_size != size => Some(IFrameSizeMsg {
-                        browsing_context_id: *browsing_context_id,
-                        size: *size,
-                        type_: WindowSizeType::Resize,
-                    }),
-                    None => Some(IFrameSizeMsg {
-                        browsing_context_id: *browsing_context_id,
-                        size: *size,
-                        type_: WindowSizeType::Initial,
-                    }),
-                    _ => None,
-                }
-            })
-            .collect();
-
-        if !size_messages.is_empty() {
-            let msg = ConstellationMsg::IFrameSizes(size_messages);
-            if let Err(e) = self.constellation_chan.send(msg) {
-                warn!("Layout resize to constellation failed ({}).", e);
-            }
-        }
-    }
-
     fn viewport_did_change(&mut self, window_size_data: WindowSizeData) -> bool {
         let new_pixel_ratio = window_size_data.device_pixel_ratio.get();
         let new_viewport_size = Size2D::new(
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index 0e57e4ad26c..f12f7daea40 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -196,7 +196,7 @@ impl HTMLIFrameElement {
 
         let window_size = WindowSizeData {
             initial_viewport: window
-                .inner_window_dimensions_query(browsing_context_id, can_gc)
+                .get_iframe_size_if_known(browsing_context_id, can_gc)
                 .unwrap_or_default(),
             device_pixel_ratio: window.device_pixel_ratio(),
         };
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 833ad837bf1..1c9f755c61a 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -51,14 +51,14 @@ use profile_traits::ipc as ProfiledIpc;
 use profile_traits::mem::ProfilerChan as MemProfilerChan;
 use profile_traits::time::ProfilerChan as TimeProfilerChan;
 use script_layout_interface::{
-    combine_id_with_fragment_type, FragmentType, Layout, PendingImageState, QueryMsg, Reflow,
-    ReflowGoal, ReflowRequest, TrustedNodeAddress,
+    combine_id_with_fragment_type, FragmentType, IFrameSizes, Layout, PendingImageState, QueryMsg,
+    Reflow, ReflowGoal, ReflowRequest, TrustedNodeAddress,
 };
 use script_traits::webdriver_msg::{WebDriverJSError, WebDriverJSResult};
 use script_traits::{
-    ConstellationControlMsg, DocumentState, HistoryEntryReplacement, LoadData, ScriptMsg,
-    ScriptToConstellationChan, ScrollState, StructuredSerializedData, Theme, TimerSchedulerMsg,
-    WindowSizeData, WindowSizeType,
+    ConstellationControlMsg, DocumentState, HistoryEntryReplacement, IFrameSizeMsg, LoadData,
+    ScriptMsg, ScriptToConstellationChan, ScrollState, StructuredSerializedData, Theme,
+    TimerSchedulerMsg, WindowSizeData, WindowSizeType,
 };
 use selectors::attr::CaseSensitivity;
 use servo_arc::Arc as ServoArc;
@@ -377,6 +377,15 @@ pub struct Window {
 
     /// <https://dom.spec.whatwg.org/#window-current-event>
     current_event: DomRefCell<Option<Dom<Event>>>,
+
+    /// Sizes of the various `<iframes>` that we might have on this [`Window`].
+    /// This is used to:
+    ///  - Send the proper size for the `<iframe>` during new-Pipeline creation
+    ///    when the `src` attribute changes.
+    ///  - Let the `Constellation` know about `BrowsingContext` (one per `<iframe>`)
+    ///    size changes when an `<iframe>` changes size during layout.
+    #[no_trace]
+    iframe_sizes: RefCell<IFrameSizes>,
 }
 
 impl Window {
@@ -1958,12 +1967,48 @@ impl Window {
             }
         }
 
+        self.handle_new_iframe_sizes_after_layout(results.iframe_sizes);
+
         document.update_animations_post_reflow();
         self.update_constellation_epoch();
 
         true
     }
 
+    /// Update the recorded iframe sizes of the contents of layout. When these sizes change,
+    /// send a message to the `Constellation` informing it of the new sizes.
+    fn handle_new_iframe_sizes_after_layout(&self, new_iframe_sizes: IFrameSizes) {
+        let old_iframe_sizes = self.iframe_sizes.replace(new_iframe_sizes);
+        let new_iframe_sizes = self.iframe_sizes.borrow();
+        if new_iframe_sizes.is_empty() {
+            return;
+        }
+
+        // Send asynchronous updates to `Constellation.`
+        let size_messages: Vec<_> = new_iframe_sizes
+            .iter()
+            .filter_map(|(browsing_context_id, size)| {
+                match old_iframe_sizes.get(browsing_context_id) {
+                    Some(old_size) if old_size.size == size.size => None,
+                    Some(..) => Some(IFrameSizeMsg {
+                        browsing_context_id: *browsing_context_id,
+                        size: size.size,
+                        type_: WindowSizeType::Resize,
+                    }),
+                    None => Some(IFrameSizeMsg {
+                        browsing_context_id: *browsing_context_id,
+                        size: size.size,
+                        type_: WindowSizeType::Initial,
+                    }),
+                }
+            })
+            .collect();
+
+        if !size_messages.is_empty() {
+            self.send_to_constellation(ScriptMsg::IFrameSizes(size_messages));
+        }
+    }
+
     /// Reflows the page if it's possible to do so and the page is dirty. Returns true if layout
     /// actually happened, false otherwise.
     ///
@@ -2255,17 +2300,20 @@ impl Window {
         ))
     }
 
-    pub fn inner_window_dimensions_query(
+    /// If the given |browsing_context_id| refers to an `<iframe>` that is an element
+    /// in this [`Window`] and that `<iframe>` has been laid out, return its size.
+    /// Otherwise, return `None`.
+    pub(crate) fn get_iframe_size_if_known(
         &self,
-        browsing_context: BrowsingContextId,
+        browsing_context_id: BrowsingContextId,
         can_gc: CanGc,
     ) -> Option<Size2D<f32, CSSPixel>> {
-        if !self.layout_reflow(QueryMsg::InnerWindowDimensionsQuery, can_gc) {
-            return None;
-        }
-        self.layout
+        // Reflow might fail, but do a best effort to return the right size.
+        self.layout_reflow(QueryMsg::InnerWindowDimensionsQuery, can_gc);
+        self.iframe_sizes
             .borrow()
-            .query_inner_window_dimension(browsing_context)
+            .get(&browsing_context_id)
+            .map(|iframe_size| iframe_size.size)
     }
 
     #[allow(unsafe_code)]
@@ -2800,6 +2848,7 @@ impl Window {
             layout_marker: DomRefCell::new(Rc::new(Cell::new(true))),
             current_event: DomRefCell::new(None),
             theme: Cell::new(PrefersColorScheme::Light),
+            iframe_sizes: RefCell::default(),
         });
 
         unsafe { WindowBinding::Wrap(JSContext::from_ptr(runtime.cx()), win) }
diff --git a/components/shared/script/script_msg.rs b/components/shared/script/script_msg.rs
index 75da73fa167..9e51cf6d548 100644
--- a/components/shared/script/script_msg.rs
+++ b/components/shared/script/script_msg.rs
@@ -46,8 +46,6 @@ pub struct IFrameSizeMsg {
 /// Messages from the layout to the constellation.
 #[derive(Deserialize, Serialize)]
 pub enum LayoutMsg {
-    /// Inform the constellation of the size of the iframe's viewport.
-    IFrameSizes(Vec<IFrameSizeMsg>),
     /// Requests that the constellation inform the compositor that it needs to record
     /// the time when the frame with the given ID (epoch) is painted.
     PendingPaintMetric(PipelineId, Epoch),
@@ -57,7 +55,6 @@ impl fmt::Debug for LayoutMsg {
     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
         use self::LayoutMsg::*;
         let variant = match *self {
-            IFrameSizes(..) => "IFrameSizes",
             PendingPaintMetric(..) => "PendingPaintMetric",
         };
         write!(formatter, "LayoutMsg::{}", variant)
@@ -260,6 +257,8 @@ pub enum ScriptMsg {
     GetWebGPUChan(IpcSender<Option<WebGPU>>),
     /// Notify the constellation of a pipeline's document's title.
     TitleChanged(PipelineId, String),
+    /// Notify the constellation that the size of some `<iframe>`s has changed.
+    IFrameSizes(Vec<IFrameSizeMsg>),
 }
 
 impl fmt::Debug for ScriptMsg {
@@ -320,6 +319,7 @@ impl fmt::Debug for ScriptMsg {
             #[cfg(feature = "webgpu")]
             GetWebGPUChan(..) => "GetWebGPUChan",
             TitleChanged(..) => "TitleChanged",
+            IFrameSizes(..) => "IFramSizes",
         };
         write!(formatter, "ScriptMsg::{}", variant)
     }
diff --git a/components/shared/script_layout/Cargo.toml b/components/shared/script_layout/Cargo.toml
index 5de7c95f9c7..1dee24a3107 100644
--- a/components/shared/script_layout/Cargo.toml
+++ b/components/shared/script_layout/Cargo.toml
@@ -18,6 +18,7 @@ atomic_refcell = { workspace = true }
 canvas_traits = { workspace = true }
 crossbeam-channel = { workspace = true }
 euclid = { workspace = true }
+fnv = { workspace = true }
 fonts = { path = "../../fonts" }
 fonts_traits = { workspace = true }
 html5ever = { workspace = true }
diff --git a/components/shared/script_layout/lib.rs b/components/shared/script_layout/lib.rs
index 591e356320c..f8643e866e7 100644
--- a/components/shared/script_layout/lib.rs
+++ b/components/shared/script_layout/lib.rs
@@ -23,6 +23,7 @@ use base::Epoch;
 use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use euclid::default::{Point2D, Rect};
 use euclid::Size2D;
+use fnv::FnvHashMap;
 use fonts::SystemFontServiceProxy;
 use ipc_channel::ipc::IpcSender;
 use libc::c_void;
@@ -257,10 +258,6 @@ pub trait Layout {
     fn query_content_boxes(&self, node: OpaqueNode) -> Vec<Rect<Au>>;
     fn query_client_rect(&self, node: OpaqueNode) -> Rect<i32>;
     fn query_element_inner_outer_text(&self, node: TrustedNodeAddress) -> String;
-    fn query_inner_window_dimension(
-        &self,
-        context: BrowsingContextId,
-    ) -> Option<Size2D<f32, CSSPixel>>;
     fn query_nodes_from_point(
         &self,
         point: Point2D<f32>,
@@ -400,11 +397,24 @@ pub struct Reflow {
     pub page_clip_rect: Rect<Au>,
 }
 
+#[derive(Clone, Debug, MallocSizeOf)]
+pub struct IFrameSize {
+    pub browsing_context_id: BrowsingContextId,
+    pub pipeline_id: PipelineId,
+    pub size: Size2D<f32, CSSPixel>,
+}
+
+pub type IFrameSizes = FnvHashMap<BrowsingContextId, IFrameSize>;
+
 /// Information derived from a layout pass that needs to be returned to the script thread.
 #[derive(Debug, Default)]
 pub struct ReflowResult {
     /// The list of images that were encountered that are in progress.
     pub pending_images: Vec<PendingImage>,
+    /// The list of iframes in this layout and their sizes, used in order
+    /// to communicate them with the Constellation and also the `Window`
+    /// element of their content pages.
+    pub iframe_sizes: IFrameSizes,
 }
 
 /// Information needed for a script-initiated reflow.