Don't scroll background and borders of overflow:scroll blocks

We want them to be children of the parent scrolling root instead of the
scrolling root of the particular block they belong to. Also, we adjust
the scroll root to only include the content region.

components/layout/display_list_builder.rs

@@ -15,7 +15,6 @@ use block::{BlockFlow, BlockStackingContextType};
 use canvas_traits::{CanvasData, CanvasMsg, FromLayoutMsg};
 use context::SharedLayoutContext;
 use euclid::{Matrix4D, Point2D, Radians, Rect, SideOffsets2D, Size2D};
-use euclid::point::TypedPoint2D;
 use flex::FlexFlow;
 use flow::{BaseFlow, Flow, IS_ABSOLUTELY_POSITIONED};
 use flow_ref::FlowRef;
@@ -98,6 +97,7 @@ pub struct DisplayListBuildState<'a> {
     pub scroll_roots: HashMap<ScrollRootId, ScrollRoot>,
     pub stacking_context_id_stack: Vec<StackingContextId>,
     pub scroll_root_id_stack: Vec<ScrollRootId>,
+    pub processing_scroll_root_element: bool,
 }
 
 impl<'a> DisplayListBuildState<'a> {
@@ -111,6 +111,7 @@ impl<'a> DisplayListBuildState<'a> {
             scroll_roots: HashMap::new(),
             stacking_context_id_stack: vec!(stacking_context_id),
             scroll_root_id_stack: vec!(ScrollRootId::root()),
+            processing_scroll_root_element: false,
         }
     }
 
@@ -162,6 +163,14 @@ impl<'a> DisplayListBuildState<'a> {
                                 cursor: Option<Cursor>,
                                 section: DisplayListSection)
                                 -> BaseDisplayItem {
+        let scroll_root_id = if (section == DisplayListSection::BackgroundAndBorders ||
+                                 section == DisplayListSection::BlockBackgroundsAndBorders) &&
+                                 self.processing_scroll_root_element {
+            self.parent_scroll_root_id()
+        } else {
+            self.scroll_root_id()
+        };
+
         BaseDisplayItem::new(&bounds,
                              DisplayItemMetadata {
                                  node: node,
@@ -170,7 +179,7 @@ impl<'a> DisplayListBuildState<'a> {
                              &clip,
                              section,
                              self.stacking_context_id(),
-                             self.scroll_root_id())
+                             scroll_root_id)
     }
 
     pub fn to_display_list(mut self) -> DisplayList {
@@ -1977,21 +1986,25 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
 
         if self.has_scrolling_overflow() {
             let size = self.base.overflow.scroll.size;
-            let mut clip = self.fragment.stacking_relative_border_box(
+            let coordinate_system = if establishes_stacking_context {
+                CoordinateSystem::Own
+            } else {
+                CoordinateSystem::Parent
+            };
+
+            let border_box = self.fragment.stacking_relative_border_box(
                 &self.base.stacking_relative_position,
                 &self.base.early_absolute_position_info.relative_containing_block_size,
                 self.base.early_absolute_position_info.relative_containing_block_mode,
-                CoordinateSystem::Parent);
-            if establishes_stacking_context {
-                clip = Rect::new(TypedPoint2D::zero(), clip.size);
-            }
+                coordinate_system);
 
             let parent_id = state.parent_scroll_root_id();
+            state.processing_scroll_root_element = true;
             state.add_scroll_root(
                 ScrollRoot {
                     id: self.base.scroll_root_id,
                     parent_id: parent_id,
-                    clip: clip,
+                    clip: self.fragment.stacking_relative_content_box(&border_box),
                     size: size,
                 }
             );
@@ -2012,6 +2025,8 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                                 &self.base.clip);
 
         self.base.build_display_items_for_debugging_tint(state, self.fragment.node);
+
+        state.processing_scroll_root_element = false;
     }
 
     fn switch_coordinate_system_if_necessary(&mut self) {

tests/wpt/mozilla/meta/MANIFEST.json

@@ -6414,6 +6414,18 @@
             "url": "/_mozilla/mozilla/iframe/resize_after_load.html"
           }
         ],
+        "mozilla/scrolling_div_background_borders.html": [
+          {
+            "path": "mozilla/scrolling_div_background_borders.html",
+            "references": [
+              [
+                "/_mozilla/mozilla/scrolling_div_background_borders_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/mozilla/scrolling_div_background_borders.html"
+          }
+        ],
         "mozilla/simple_scroll_to_fragment.html": [
           {
             "path": "mozilla/simple_scroll_to_fragment.html",
@@ -21444,6 +21456,18 @@
           "url": "/_mozilla/mozilla/iframe/resize_after_load.html"
         }
       ],
+      "mozilla/scrolling_div_background_borders.html": [
+        {
+          "path": "mozilla/scrolling_div_background_borders.html",
+          "references": [
+            [
+              "/_mozilla/mozilla/scrolling_div_background_borders_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/mozilla/scrolling_div_background_borders.html"
+        }
+      ],
       "mozilla/simple_scroll_to_fragment.html": [
         {
           "path": "mozilla/simple_scroll_to_fragment.html",

tests/wpt/mozilla/tests/mozilla/scrolling_div_background_borders.html

@@ -0,0 +1,25 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Background and borders of scrolling div should not scroll</title>
+<link rel="match" href="scrolling_div_background_borders_ref.html">
+<body>
+    <style>
+        #scroller {
+            background: url('scrolling_div_background_borders_background.png'), green;
+            height: 100px;
+            width: 100px;
+            overflow: scroll;
+            border: solid blue 10px;
+        }
+    </style>
+
+    <!-- We are testing that background and borders don't scroll with the content
+         and that the content doesn't overlap the borders when it scrolls. -->
+    <div id="scroller">
+        <div style="width: 100px; height: 200px; background: green;"></div>
+    </div>
+
+    <script>
+            document.getElementById("scroller").scrollTop = 100;
+    </script>
+</body>

tests/wpt/mozilla/tests/mozilla/scrolling_div_background_borders_ref.html

@@ -0,0 +1,13 @@
+<!doctype html>
+<meta charset="utf-8">
+<body>
+    <style>
+        #box {
+            background: green;
+            height: 100px;
+            width: 100px;
+            border: solid blue 10px;
+        }
+    </style>
+    <div id="box"></div>
+</body>