layout: Allow non-absolutely-positioned elements with `overflow: scroll`

set to be scrolled.

This makes them establish stacking contexts, which is a CSS 2.1 spec
violation. However, we were already violating the spec here for
absolutely-positioned elements with `overflow: scroll`. It will probably
be easier to fix this spec violation once we either switch entirely to
WebRender or we have multiple layers per stacking context.

Closes #2742.

4 files changed, 54 insertions, 27 deletions

diff --git a/components/gfx_traits/lib.rs b/components/gfx_traits/lib.rs
index 5833e847a91..6f5abcb1963 100644
--- a/components/gfx_traits/lib.rs
+++ b/components/gfx_traits/lib.rs
@@ -105,6 +105,10 @@ impl LayerId {
         let LayerId(layer_type, id, _) = *self;
         LayerId(layer_type, id, 0)
     }
+
+    pub fn kind(&self) -> LayerType {
+        self.0
+    }
 }
 
 /// All layer-specific information that the painting task sends to the compositor other than the
diff --git a/components/layout/block.rs b/components/layout/block.rs
index 390810e9831..519a76c08e7 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -1563,10 +1563,6 @@ impl BlockFlow {
     }
 
     pub fn has_scrolling_overflow(&self) -> bool {
-        if !self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
-            return false;
-        }
-
         match (self.fragment.style().get_box().overflow_x,
                self.fragment.style().get_box().overflow_y.0) {
             (overflow_x::T::auto, _) | (overflow_x::T::scroll, _) |
diff --git a/components/layout/display_list_builder.rs b/components/layout/display_list_builder.rs
index 7f8fe5df579..0c04c839450 100644
--- a/components/layout/display_list_builder.rs
+++ b/components/layout/display_list_builder.rs
@@ -1470,33 +1470,22 @@ impl FragmentDisplayListBuilding for Fragment {
 
         // Clip according to the values of `overflow-x` and `overflow-y`.
         //
-        // TODO(pcwalton): Support scrolling of non-absolutely-positioned elements.
         // FIXME(pcwalton): This may be more complex than it needs to be, since it seems to be
         // impossible with the computed value rules as they are to have `overflow-x: visible` with
         // `overflow-y: <scrolling>` or vice versa!
-        match (self.style.get_box().overflow_x, is_absolutely_positioned) {
-            (overflow_x::T::hidden, _) |
-            (overflow_x::T::auto, false) |
-            (overflow_x::T::scroll, false) => {
-                let mut bounds = current_clip.bounding_rect();
-                let max_x = cmp::min(bounds.max_x(), overflow_clip_rect.max_x());
-                bounds.origin.x = cmp::max(bounds.origin.x, overflow_clip_rect.origin.x);
-                bounds.size.width = max_x - bounds.origin.x;
-                current_clip.intersect_rect(&bounds)
-            }
-            _ => {}
+        if let overflow_x::T::hidden = self.style.get_box().overflow_x {
+            let mut bounds = current_clip.bounding_rect();
+            let max_x = cmp::min(bounds.max_x(), overflow_clip_rect.max_x());
+            bounds.origin.x = cmp::max(bounds.origin.x, overflow_clip_rect.origin.x);
+            bounds.size.width = max_x - bounds.origin.x;
+            current_clip.intersect_rect(&bounds)
         }
-        match (self.style.get_box().overflow_y.0, is_absolutely_positioned) {
-            (overflow_x::T::hidden, _) |
-            (overflow_x::T::auto, false) |
-            (overflow_x::T::scroll, false) => {
-                let mut bounds = current_clip.bounding_rect();
-                let max_y = cmp::min(bounds.max_y(), overflow_clip_rect.max_y());
-                bounds.origin.y = cmp::max(bounds.origin.y, overflow_clip_rect.origin.y);
-                bounds.size.height = max_y - bounds.origin.y;
-                current_clip.intersect_rect(&bounds)
-            }
-            _ => {}
+        if let overflow_x::T::hidden = self.style.get_box().overflow_y.0 {
+            let mut bounds = current_clip.bounding_rect();
+            let max_y = cmp::min(bounds.max_y(), overflow_clip_rect.max_y());
+            bounds.origin.y = cmp::max(bounds.origin.y, overflow_clip_rect.origin.y);
+            bounds.size.height = max_y - bounds.origin.y;
+            current_clip.intersect_rect(&bounds)
         }
 
         let border_radii = build_border_radius(stacking_relative_border_box,
diff --git a/tests/html/overflow_scroll_relative_position.html b/tests/html/overflow_scroll_relative_position.html
new file mode 100644
index 00000000000..83c4b8c1675
--- /dev/null
+++ b/tests/html/overflow_scroll_relative_position.html
@@ -0,0 +1,38 @@
+<html>
+  <head>
+    <link rel=match href=overflow_simple_b.html>
+    <style>
+      #first {
+      height: 100px;
+      width: 100px;
+      overflow: scroll;
+      position: relative;
+      }
+      #second {
+      position: absolute;
+      height: 100px;
+      width: 90px;
+      top: 0;
+      left: 0;
+      background: green;
+      }
+      #third {
+      position: absolute;
+      height: 100px;
+      width: 110px;
+      top: 0;
+      left: 90px;
+      background: orange;
+      }
+    </style>
+  </head>
+  <body>
+    This element should be scrollable.
+    <div id="first">
+      <div id="second">
+      </div>
+      <div id="third">
+      </div>
+    </div>
+  </body>
+</html>