Bug 1519007 - Do not allow APZ to move the layout viewport outside the scrollable rect. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D24826

--HG--
extra : moz-landing-system : lando

gfx/layers/FrameMetrics.cpp

@@ -18,12 +18,13 @@ void FrameMetrics::RecalculateLayoutViewportOffset() {
     return;
   }
   KeepLayoutViewportEnclosingVisualViewport(GetVisualViewport(),
-                                            mLayoutViewport);
+                                            mScrollableRect, mLayoutViewport);
 }
 
 /* static */
 void FrameMetrics::KeepLayoutViewportEnclosingVisualViewport(
-    const CSSRect& aVisualViewport, CSSRect& aLayoutViewport) {
+    const CSSRect& aVisualViewport, const CSSRect& aScrollableRect,
+    CSSRect& aLayoutViewport) {
   // If the visual viewport is contained within the layout viewport, we don't
   // need to make any adjustments, so we can exit early.
   //
@@ -77,6 +78,10 @@ void FrameMetrics::KeepLayoutViewportEnclosingVisualViewport(
                               aLayoutViewport.YMost());
     }
   }
+
+  // Regardless of any adjustment above, the layout viewport is not allowed
+  // to go outside the scrollable rect.
+  aLayoutViewport = aLayoutViewport.MoveInsideAndClamp(aScrollableRect);
 }
 
 void ScrollMetadata::SetUsesContainerScrolling(bool aValue) {

gfx/layers/FrameMetrics.h

@@ -511,8 +511,12 @@ struct FrameMetrics {
   // the frame delay).
   // Modifies |aLayoutViewport| to continue enclosing |aVisualViewport|
   // if possible.
+  // The layout viewport needs to remain clamped to the scrollable rect,
+  // and we pass in the scrollable rect so this function can maintain that
+  // constraint.
   static void KeepLayoutViewportEnclosingVisualViewport(
-      const CSSRect& aVisualViewport, CSSRect& aLayoutViewport);
+      const CSSRect& aVisualViewport, const CSSRect& aScrollableRect,
+      CSSRect& aLayoutViewport);
 
  private:
   // A ID assigned to each scrollable frame, unique within each LayersId..

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3545,7 +3545,7 @@ void AsyncPanZoomController::AdjustScrollForSurfaceShift(
   FrameMetrics::KeepLayoutViewportEnclosingVisualViewport(
       CSSRect(mCompositedScrollOffset,
               Metrics().CalculateCompositedSizeInCssPixels()),
-      mCompositedLayoutViewport);
+      Metrics().GetScrollableRect(), mCompositedLayoutViewport);
   RequestContentRepaint();
   UpdateSharedCompositorFrameMetrics();
 }