Handle nested position:sticky elements correctly (compositor)

third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h

Index: third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h
diff --git a/third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h b/third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h
index 296496d76f24a22d2545fa38c5d5c6555e31f2ef..1c3dd455a357bc9a210d096cf0977cd744036005 100644
--- a/third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h
+++ b/third_party/WebKit/public/platform/WebLayerStickyPositionConstraint.h
@@ -26,8 +26,9 @@
 #ifndef WebLayerStickyPositionConstraint_h
 #define WebLayerStickyPositionConstraint_h
 
-#include "public/platform/WebPoint.h"
-#include "public/platform/WebRect.h"
+#include "public/platform/WebFloatPoint.h"
+#include "public/platform/WebFloatRect.h"
+#include "public/platform/WebLayer.h"
 
 namespace blink {
 
@@ -52,15 +53,20 @@ struct WebLayerStickyPositionConstraint {
 
   // This is the layout position of the sticky element before it has been
   // shifted relative to the enclosing composited layer.
-  WebPoint parentRelativeStickyBoxOffset;
+  WebFloatPoint parentRelativeStickyBoxOffset;

[chrishtr, 2017/03/20 17:24:52]

Are these now floating-point to account for adding subpixel sticky offsets? If
so,
how is snapping done in the compositor?

[smcgruer, 2017/03/21 13:54:55]

This came about due to flackr's concerns about losing precision in
total_sticky_box_sticky_offset and total_containing_block_sticky_offset in
patchset 5. Once those were float, there was then a natural progression to clean
things up, including parentRelativeStickyBoxOffset which was created from a
float (the old stickyBoxOffset in CompositedLayerMapping) and was used to set a
Vector2dF (main_thread_offset).

I guess there is a question here as to whether it is correct to remove the
rounding that was previously applied to parentRelativeStickyBoxOffset via
'roundedIntPoint'?

(The output of the cc side is and always was a Vector2dF, which is incorporated
as a translate Transform).

[smcgruer, 2017/03/21 16:01:25]

Discussed with flackr. I overreached in my cleanup attempts. :D. Will move the
parentRelativeStickyBoxOffset back to WebPoint.

[smcgruer, 2017/03/21 17:37:48]

Done.

 
   // The layout rectangle of the sticky element before it has been shifted
   // to stick.
-  WebRect scrollContainerRelativeStickyBoxRect;
+  WebFloatRect scrollContainerRelativeStickyBoxRect;
 
   // The layout rectangle of the containing block edges which this sticky
   // element should not be shifted beyond.
-  WebRect scrollContainerRelativeContainingBlockRect;
+  WebFloatRect scrollContainerRelativeContainingBlockRect;
+
+  // The nearest ancestor sticky layers that affect the sticky box constraint
+  // rect and the containing block constraint rect respectively.
+  int nearestLayerShiftingStickyBox;
+  int nearestLayerShiftingContainingBlock;
 
   WebLayerStickyPositionConstraint()
       : isSticky(false),
@@ -71,7 +77,9 @@ struct WebLayerStickyPositionConstraint {
         leftOffset(0.f),
         rightOffset(0.f),
         topOffset(0.f),
-        bottomOffset(0.f) {}
+        bottomOffset(0.f),
+        nearestLayerShiftingStickyBox(WebLayer::kInvalidLayerId),
+        nearestLayerShiftingContainingBlock(WebLayer::kInvalidLayerId) {}
 };
 
 }  // namespace blink