Only create scrolling overflow regions when necessary

Only create scroll roots for overflow regions when the overflow region
is actually larger than the container size. This prevents creating
scrolling roots for elements that do not have overflow scroll as a
side-effect of the way their height and width is defined. For example,
tables should never respect overflow:scroll since their height and
width should always be large enough to prevent overflow. This also
decreases the size and complexity of the display list in many other
circumstances.

As part of this change, transformed overflow calculation is moved from
display list construction to layout. This should mean that overflow is
handled more accurately earlier.

Fixes #14574.

Cargo.lock

@@ -1330,6 +1330,7 @@ dependencies = [
  "serde_json 0.8.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "servo_atoms 0.0.1",
  "servo_config 0.0.1",
+ "servo_geometry 0.0.1",
  "servo_url 0.0.1",
  "smallvec 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "style 0.0.1",

components/gfx/display_list/mod.rs

@@ -25,7 +25,7 @@ use ipc_channel::ipc::IpcSharedMemory;
 use msg::constellation_msg::PipelineId;
 use net_traits::image::base::{Image, PixelFormat};
 use range::Range;
-use servo_geometry::{au_rect_to_f32_rect, f32_rect_to_au_rect, max_rect};
+use servo_geometry::max_rect;
 use std::cmp::{self, Ordering};
 use std::collections::HashMap;
 use std::fmt;
@@ -413,24 +413,6 @@ impl StackingContext {
                              ScrollRootId::root())
     }
 
-    pub fn overflow_rect_in_parent_space(&self) -> Rect<Au> {
-        // Transform this stacking context to get it into the same space as
-        // the parent stacking context.
-        //
-        // TODO: Take into account 3d transforms, even though it's a fairly
-        // uncommon case.
-        let origin_x = self.bounds.origin.x.to_f32_px();
-        let origin_y = self.bounds.origin.y.to_f32_px();
-
-        let transform = Matrix4D::identity().pre_translated(origin_x, origin_y, 0.0)
-                                            .pre_mul(&self.transform);
-        let transform_2d = transform.to_2d();
-
-        let overflow = au_rect_to_f32_rect(self.overflow);
-        let overflow = transform_2d.transform_rect(&overflow);
-        f32_rect_to_au_rect(overflow)
-    }
-
     pub fn to_display_list_items(self) -> (DisplayItem, DisplayItem) {
         let mut base_item = BaseDisplayItem::empty();
         base_item.stacking_context_id = self.id;

components/layout/Cargo.toml

@@ -38,6 +38,7 @@ script_traits = {path = "../script_traits"}
 selectors = "0.15"
 serde = "0.8"
 serde_derive = "0.8"
+servo_geometry = {path = "../geometry"}
 serde_json = "0.8"
 servo_atoms = {path = "../atoms"}
 servo_config = {path = "../config"}

components/layout/block.rs

@@ -518,6 +518,8 @@ bitflags! {
     flags BlockFlowFlags: u8 {
         #[doc = "If this is set, then this block flow is the root flow."]
         const IS_ROOT = 0b0000_0001,
+        #[doc = "If this is set, then this block flow has overflow and it will scroll."]
+        const HAS_SCROLLING_OVERFLOW = 0b0000_0010,
     }
 }
 
@@ -1675,7 +1677,7 @@ impl BlockFlow {
         }
     }
 
-    pub fn has_scrolling_overflow(&self) -> bool {
+    pub fn style_permits_scrolling_overflow(&self) -> bool {
         match (self.fragment.style().get_box().overflow_x,
                self.fragment.style().get_box().overflow_y.0) {
             (overflow_x::T::auto, _) | (overflow_x::T::scroll, _) |
@@ -1823,6 +1825,19 @@ impl BlockFlow {
                                               Au::from_f32_px(clip_rect.size.height)));
         self.base.clip = ClippingRegion::from_rect(&clip_rect)
     }
+
+    pub fn mark_scrolling_overflow(&mut self, has_scrolling_overflow: bool) {
+        if has_scrolling_overflow {
+            self.flags.insert(HAS_SCROLLING_OVERFLOW);
+        } else {
+            self.flags.remove(HAS_SCROLLING_OVERFLOW);
+        }
+    }
+
+    pub fn has_scrolling_overflow(&mut self) -> bool {
+        self.flags.contains(HAS_SCROLLING_OVERFLOW)
+    }
+
 }
 
 impl Flow for BlockFlow {

components/layout/display_list_builder.rs

@@ -128,14 +128,7 @@ impl<'a> DisplayListBuildState<'a> {
 
     fn add_stacking_context(&mut self,
                             parent_id: StackingContextId,
-                            mut stacking_context: StackingContext) {
-        self.update_overflow_for_stacking_context(&mut stacking_context);
-        self.add_stacking_context_without_calcuating_overflow(parent_id, stacking_context);
-    }
-
-    fn add_stacking_context_without_calcuating_overflow(&mut self,
-                                                        parent_id: StackingContextId,
-                                                        stacking_context: StackingContext) {
+                            stacking_context: StackingContext) {
         let contexts = self.stacking_context_children.entry(parent_id).or_insert(Vec::new());
         contexts.push(stacking_context);
     }
@@ -194,24 +187,6 @@ impl<'a> DisplayListBuildState<'a> {
         }
     }
 
-    fn update_overflow_for_stacking_context(&mut self, stacking_context: &mut StackingContext) {
-        if stacking_context.context_type != StackingContextType::Real {
-            return;
-        }
-
-        let children = self.stacking_context_children.get_mut(&stacking_context.id);
-        if let Some(children) = children {
-            for child in children {
-                if child.context_type == StackingContextType::Real {
-                    // This child might be transformed, so we need to take into account
-                    // its transformed overflow rect too, but at the correct position.
-                    let overflow = child.overflow_rect_in_parent_space();
-                    stacking_context.overflow = stacking_context.overflow.union(&overflow);
-                }
-            }
-        }
-    }
-
     fn to_display_list_for_stacking_context(&mut self,
                                             list: &mut Vec<DisplayItem>,
                                             stacking_context: StackingContext,
@@ -1842,12 +1817,10 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
     }
 
     fn collect_scroll_root_for_block(&mut self, state: &mut DisplayListBuildState) -> ScrollRootId {
-        if !self.has_scrolling_overflow() {
+        if !self.style_permits_scrolling_overflow() {
             return state.current_scroll_root_id;
         }
 
-        let scroll_root_id = ScrollRootId::new_of_type(self.fragment.node.id() as usize,
-                                                       self.fragment.fragment_type());
         let coordinate_system = if self.fragment.establishes_stacking_context() {
             CoordinateSystem::Own
         } else {
@@ -1859,13 +1832,24 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
             &self.base.early_absolute_position_info.relative_containing_block_size,
             self.base.early_absolute_position_info.relative_containing_block_mode,
             coordinate_system);
+        let clip = self.fragment.stacking_relative_content_box(&border_box);
 
+        let has_scrolling_overflow = self.base.overflow.scroll.origin != Point2D::zero() ||
+                                     self.base.overflow.scroll.size.width > clip.size.width ||
+                                     self.base.overflow.scroll.size.height > clip.size.height;
+        self.mark_scrolling_overflow(has_scrolling_overflow);
+        if !has_scrolling_overflow {
+            return state.current_scroll_root_id;
+        }
+
+        let scroll_root_id = ScrollRootId::new_of_type(self.fragment.node.id() as usize,
+                                                       self.fragment.fragment_type());
         let parent_scroll_root_id = state.current_scroll_root_id;
         state.add_scroll_root(
             ScrollRoot {
                 id: scroll_root_id,
                 parent_id: parent_scroll_root_id,
-                clip: self.fragment.stacking_relative_content_box(&border_box),
+                clip: clip,
                 size: self.base.overflow.scroll.size,
             }
         );
@@ -1898,10 +1882,9 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
             state.stacking_context_children.remove(&stacking_context_id).unwrap_or_else(Vec::new);
         for child in new_children {
             if child.context_type == StackingContextType::PseudoFloat {
-                state.add_stacking_context_without_calcuating_overflow(stacking_context_id, child);
+                state.add_stacking_context(stacking_context_id, child);
             } else {
-                state.add_stacking_context_without_calcuating_overflow(parent_stacking_context_id,
-                                                                       child);
+                state.add_stacking_context(parent_stacking_context_id, child);
             }
         }
     }
@@ -1942,9 +1925,7 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
             DisplayListSection::BlockBackgroundsAndBorders
         };
 
-        if self.has_scrolling_overflow() {
-            state.processing_scroll_root_element = true;
-        }
+        state.processing_scroll_root_element = self.has_scrolling_overflow();
 
         // Add the box that starts the block context.
         self.fragment

components/layout/flow.rs

@@ -43,6 +43,7 @@ use model::{CollapsibleMargins, IntrinsicISizes, MarginCollapseInfo};
 use multicol::MulticolFlow;
 use parallel::FlowParallelInfo;
 use serde::{Serialize, Serializer};
+use servo_geometry::{au_rect_to_f32_rect, f32_rect_to_au_rect};
 use std::{fmt, mem, raw};
 use std::iter::Zip;
 use std::slice::IterMut;
@@ -248,6 +249,46 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
         might_have_floats_in_or_out
     }
 
+    fn get_overflow_in_parent_coordinates(&self) -> Overflow {
+        // FIXME(#2795): Get the real container size.
+        let container_size = Size2D::zero();
+        let position = base(self).position.to_physical(base(self).writing_mode, container_size);
+
+        let mut overflow = base(self).overflow;
+
+        match self.class() {
+            FlowClass::Block | FlowClass::TableCaption | FlowClass::TableCell => {}
+            _ => {
+                overflow.translate(&position.origin);
+                return overflow;
+            }
+        }
+
+        if !self.as_block().fragment.establishes_stacking_context() ||
+           self.as_block().fragment.style.get_box().transform.0.is_none() {
+            overflow.translate(&position.origin);
+            return overflow;
+        }
+
+        // TODO: Take into account 3d transforms, even though it's a fairly
+        // uncommon case.
+        let transform_2d = self.as_block().fragment.transform_matrix(&position).to_2d();
+        let transformed_overflow = Overflow {
+            paint: f32_rect_to_au_rect(transform_2d.transform_rect(
+                                       &au_rect_to_f32_rect(overflow.paint))),
+            scroll: f32_rect_to_au_rect(transform_2d.transform_rect(
+                                       &au_rect_to_f32_rect(overflow.scroll))),
+        };
+
+        // TODO: We are taking the union of the overflow and transformed overflow here, which
+        // happened implicitly in the previous version of this code. This will probably be
+        // unnecessary once we are taking into account 3D transformations above.
+        overflow.union(&transformed_overflow);
+
+        overflow.translate(&position.origin);
+        overflow
+    }
+
     ///
     /// CSS Section 11.1
     /// This is the union of rectangles of the flows for which we define the
@@ -266,17 +307,11 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
             FlowClass::Block |
             FlowClass::TableCaption |
             FlowClass::TableCell => {
-                // FIXME(#2795): Get the real container size.
-                let container_size = Size2D::zero();
-
                 let overflow_x = self.as_block().fragment.style.get_box().overflow_x;
                 let overflow_y = self.as_block().fragment.style.get_box().overflow_y;
 
                 for kid in mut_base(self).children.iter_mut() {
-                    let mut kid_overflow = base(kid).overflow;
-                    let kid_position = base(kid).position.to_physical(base(kid).writing_mode,
-                                                                      container_size);
-                    kid_overflow.translate(&kid_position.origin);
+                    let mut kid_overflow = kid.get_overflow_in_parent_coordinates();
 
                     // If the overflow for this flow is hidden on a given axis, just
                     // put the existing overflow in the kid rect, so that the union

components/layout/lib.rs

@@ -50,6 +50,7 @@ extern crate serde_derive;
 extern crate serde_json;
 #[macro_use] extern crate servo_atoms;
 extern crate servo_config;
+extern crate servo_geometry;
 extern crate servo_url;
 extern crate smallvec;
 extern crate style;

tests/wpt/mozilla/meta/MANIFEST.json

@@ -5232,6 +5232,18 @@
             "url": "/_mozilla/css/table_margin_auto_a.html"
           }
         ],
+        "css/table_overflow.html": [
+          {
+            "path": "css/table_overflow.html",
+            "references": [
+              [
+                "/_mozilla/css/table_overflow_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/table_overflow.html"
+          }
+        ],
         "css/table_padding_a.html": [
           {
             "path": "css/table_padding_a.html",
@@ -20556,6 +20568,18 @@
           "url": "/_mozilla/css/table_margin_auto_a.html"
         }
       ],
+      "css/table_overflow.html": [
+        {
+          "path": "css/table_overflow.html",
+          "references": [
+            [
+              "/_mozilla/css/table_overflow_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/table_overflow.html"
+        }
+      ],
       "css/table_padding_a.html": [
         {
           "path": "css/table_padding_a.html",

tests/wpt/mozilla/tests/css/table_overflow.html

@@ -0,0 +1,19 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Ensure that table overflow:auto and overflow:scroll do not crash</title>
+<link rel="match" href="table_overflow_ref.html">
+
+<style>
+    div {
+        height: 100px;
+        width: 100px;
+        background: green;
+    }
+</style>
+
+<body>
+
+<table style="overflow: scroll;"><tr><td><div></div></td></tr></table>
+<table style="overflow: auto;"><tr><td><div></div></td></tr></table>
+
+</body>

tests/wpt/mozilla/tests/css/table_overflow_ref.html

@@ -0,0 +1,17 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Ensure that table overflow:auto and overflow:scroll do not crash</title>
+<style>
+    div {
+        height: 100px;
+        width: 100px;
+        background: green;
+    }
+</style>
+
+<body>
+
+<table><tr><td><div></div></td></tr></table>
+<table><tr><td><div></div></td></tr></table>
+
+</body>