layout: Add a ReflowPhases bitflags (#37696)

This is used to capture information about what layout phases are
necessary for a given `ReflowGoal`. It's moved closer to where these
decisions are made and it should be easier to understand what the values
mean. They had gotten a bit out of sync with how queries and layout were
implemented.

Testing: This shouldn't change observable behavior and thus should be
covered
by existing WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/layout_impl.rs

@@ -14,6 +14,7 @@ use std::sync::{Arc, LazyLock};
 use app_units::Au;
 use base::Epoch;
 use base::id::{PipelineId, WebViewId};
+use bitflags::bitflags;
 use compositing_traits::CrossProcessCompositorApi;
 use compositing_traits::display_list::ScrollType;
 use embedder_traits::{Theme, UntrustedNodeAddress, ViewportDetails};
@@ -25,8 +26,8 @@ use fonts_traits::StylesheetWebFontLoadFinishedCallback;
 use fxhash::FxHashMap;
 use ipc_channel::ipc::IpcSender;
 use layout_api::{
-    Layout, LayoutConfig, LayoutFactory, NodesFromPointQueryType, OffsetParentResponse, ReflowGoal,
-    ReflowRequest, ReflowResult, TrustedNodeAddress,
+    Layout, LayoutConfig, LayoutFactory, NodesFromPointQueryType, OffsetParentResponse, QueryMsg,
+    ReflowGoal, ReflowRequest, ReflowResult, TrustedNodeAddress,
 };
 use log::{debug, error, warn};
 use malloc_size_of::{MallocConditionalSizeOf, MallocSizeOf, MallocSizeOfOps};
@@ -588,21 +589,31 @@ impl LayoutThread {
         if reflow_request.restyle_reason.needs_restyle() {
             return false;
         }
+        // We always need to at least build a fragment tree.
+        if !self.fragment_tree.borrow().is_none() {
+            return false;
+        }
 
-        // If only the fragment tree is required, and it's up-to-date, layout is unnecessary.
-        if !reflow_request.reflow_goal.needs_display() && self.fragment_tree.borrow().is_some() {
+        // If we have a fragment tree and it's up-to-date and this reflow
+        // doesn't need more reflow results, we can skip the rest of layout.
+        let necessary_phases = ReflowPhases::necessary(&reflow_request.reflow_goal);
+        if necessary_phases.is_empty() {
             return true;
         }
 
-        // If only the stacking context tree is required, and it's up-to-date, layout is unnecessary.
-        if !reflow_request.reflow_goal.needs_display_list() &&
-            self.stacking_context_tree.borrow().is_some() &&
-            !self.need_new_stacking_context_tree.get()
-        {
-            return true;
+        // If only the stacking context tree is required, and it's up-to-date,
+        // layout is unnecessary, otherwise a layout is necessary.
+        if necessary_phases == ReflowPhases::StackingContextTreeConstruction {
+            return self.stacking_context_tree.borrow().is_some() &&
+                !self.need_new_stacking_context_tree.get();
         }
 
-        // Otherwise, the only interesting thing is whether the current display list is up-to-date.
+        // Otherwise, the only interesting thing is whether the current display
+        // list is up-to-date.
+        assert_eq!(
+            necessary_phases,
+            ReflowPhases::StackingContextTreeConstruction | ReflowPhases::DisplayListConstruction
+        );
         !self.need_new_display_list.get()
     }
 
@@ -910,8 +921,8 @@ impl LayoutThread {
     }
 
     fn build_stacking_context_tree(&self, reflow_request: &ReflowRequest, damage: RestyleDamage) {
-        if !reflow_request.reflow_goal.needs_display_list() &&
-            !reflow_request.reflow_goal.needs_display()
+        if !ReflowPhases::necessary(&reflow_request.reflow_goal)
+            .contains(ReflowPhases::StackingContextTreeConstruction)
         {
             return;
         }
@@ -973,7 +984,9 @@ impl LayoutThread {
         damage: RestyleDamage,
         layout_context: &mut LayoutContext<'_>,
     ) -> bool {
-        if !reflow_request.reflow_goal.needs_display() {
+        if !ReflowPhases::necessary(&reflow_request.reflow_goal)
+            .contains(ReflowPhases::DisplayListConstruction)
+        {
             return false;
         }
         let Some(fragment_tree) = &*self.fragment_tree.borrow() else {
@@ -1363,3 +1376,41 @@ impl Drop for SnapshotSetter<'_> {
         }
     }
 }
+
+bitflags! {
+    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+    pub struct ReflowPhases: u8 {
+        const StackingContextTreeConstruction = 1 << 0;
+        const DisplayListConstruction = 1 << 1;
+    }
+}
+
+impl ReflowPhases {
+    /// Return the necessary phases of layout for the given [`ReflowGoal`]. Note that all
+    /// [`ReflowGoals`] need the basic restyle + box tree layout + fragment tree layout,
+    /// so [`ReflowPhases::empty()`] implies that.
+    fn necessary(reflow_goal: &ReflowGoal) -> Self {
+        match reflow_goal {
+            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(..) => {
+                Self::StackingContextTreeConstruction | Self::DisplayListConstruction
+            },
+            ReflowGoal::LayoutQuery(query) => match query {
+                QueryMsg::NodesFromPointQuery => {
+                    Self::StackingContextTreeConstruction | Self::DisplayListConstruction
+                },
+                QueryMsg::ResolvedStyleQuery | QueryMsg::ScrollingAreaOrOffsetQuery => {
+                    Self::StackingContextTreeConstruction
+                },
+                QueryMsg::ClientRectQuery |
+                QueryMsg::ContentBox |
+                QueryMsg::ContentBoxes |
+                QueryMsg::ElementInnerOuterTextQuery |
+                QueryMsg::InnerWindowDimensionsQuery |
+                QueryMsg::OffsetParentQuery |
+                QueryMsg::ResolvedFontStyleQuery |
+                QueryMsg::TextIndexQuery |
+                QueryMsg::StyleQuery => Self::empty(),
+            },
+        }
+    }
+}

components/shared/layout/lib.rs

@@ -346,52 +346,6 @@ pub enum ReflowGoal {
     UpdateScrollNode(ExternalScrollId, LayoutVector2D),
 }
 
-impl ReflowGoal {
-    /// Returns true if the given ReflowQuery needs a full, up-to-date display list to
-    /// be present or false if it only needs stacking-relative positions.
-    pub fn needs_display_list(&self) -> bool {
-        match *self {
-            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(..) => true,
-            ReflowGoal::LayoutQuery(ref querymsg) => match *querymsg {
-                QueryMsg::ElementInnerOuterTextQuery |
-                QueryMsg::InnerWindowDimensionsQuery |
-                QueryMsg::NodesFromPointQuery |
-                QueryMsg::ResolvedStyleQuery |
-                QueryMsg::ScrollingAreaOrOffsetQuery |
-                QueryMsg::TextIndexQuery => true,
-                QueryMsg::ClientRectQuery |
-                QueryMsg::ContentBox |
-                QueryMsg::ContentBoxes |
-                QueryMsg::OffsetParentQuery |
-                QueryMsg::ResolvedFontStyleQuery |
-                QueryMsg::StyleQuery => false,
-            },
-        }
-    }
-
-    /// Returns true if the given ReflowQuery needs its display list send to WebRender or
-    /// false if a layout_thread display list is sufficient.
-    pub fn needs_display(&self) -> bool {
-        match *self {
-            ReflowGoal::UpdateTheRendering | ReflowGoal::UpdateScrollNode(..) => true,
-            ReflowGoal::LayoutQuery(ref querymsg) => match *querymsg {
-                QueryMsg::NodesFromPointQuery |
-                QueryMsg::TextIndexQuery |
-                QueryMsg::ElementInnerOuterTextQuery => true,
-                QueryMsg::ContentBox |
-                QueryMsg::ContentBoxes |
-                QueryMsg::ClientRectQuery |
-                QueryMsg::ScrollingAreaOrOffsetQuery |
-                QueryMsg::ResolvedStyleQuery |
-                QueryMsg::ResolvedFontStyleQuery |
-                QueryMsg::OffsetParentQuery |
-                QueryMsg::InnerWindowDimensionsQuery |
-                QueryMsg::StyleQuery => false,
-            },
-        }
-    }
-}
-
 #[derive(Clone, Debug, MallocSizeOf)]
 pub struct IFrameSize {
     pub browsing_context_id: BrowsingContextId,