Auto merge of #13820 - mrobinson:remove-nesting, r=glennw

Remove the concept of nested stacking contexts from display list builder

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- [ ] These changes fix #__ (github issue number if applicable).

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With the removal of the legacy rendering path, we don't need to worry
about nested stacking contexts. This change just pushes the
complication to the WebRender conversion step, but a later change will
remove the concept from WebRender as well.

This also helps to prepare for the introduction of ids for particular
scrolling regions, an integral part of multiple ScrollLayers per block.

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components/gfx/display_list/mod.rs

@@ -589,14 +589,14 @@ pub struct StackingContext {
     /// Whether this stacking context creates a new 3d rendering context.
     pub establishes_3d_context: bool,
 
-    /// Whether this stacking context scrolls its overflow area.
-    pub scrolls_overflow_area: bool,
-
     /// The layer info for this stacking context, if there is any.
     pub layer_info: Option<LayerInfo>,
 
     /// Children of this StackingContext.
     pub children: Vec<StackingContext>,
+
+    /// If this StackingContext scrolls its overflow area, this will contain the id.
+    pub overflow_scroll_id: Option<StackingContextId>,
 }
 
 impl StackingContext {
@@ -612,8 +612,8 @@ impl StackingContext {
                transform: Matrix4D<f32>,
                perspective: Matrix4D<f32>,
                establishes_3d_context: bool,
-               scrolls_overflow_area: bool,
-               layer_info: Option<LayerInfo>)
+               layer_info: Option<LayerInfo>,
+               scroll_id: Option<StackingContextId>)
                -> StackingContext {
         StackingContext {
             id: id,
@@ -626,9 +626,9 @@ impl StackingContext {
             transform: transform,
             perspective: perspective,
             establishes_3d_context: establishes_3d_context,
-            scrolls_overflow_area: scrolls_overflow_area,
             layer_info: layer_info,
             children: Vec::new(),
+            overflow_scroll_id: scroll_id,
         }
     }
 
@@ -648,13 +648,10 @@ impl StackingContext {
     fn update_overflow_for_all_children(&mut self) {
         for child in self.children.iter() {
             if self.context_type == StackingContextType::Real &&
-               child.context_type == StackingContextType::Real &&
-               !self.scrolls_overflow_area {
+               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();
-
+                let overflow = child.overflow_rect_in_parent_space();
                 self.overflow = self.overflow.union(&overflow);
             }
         }
@@ -740,7 +737,7 @@ impl fmt::Debug for StackingContext {
             "Pseudo-StackingContext"
         };
 
-        let scrollable_string = if self.scrolls_overflow_area {
+        let scrollable_string = if self.overflow_scroll_id.is_some() {
             " (scrolls overflow area)"
         } else {
             ""