Bug 1543485 - Preserve the relative scroll offset of the two viewports when accepting a main thread scroll update. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D87686

gfx/layers/FrameMetrics.cpp

@@ -93,6 +93,21 @@ void FrameMetrics::KeepLayoutViewportEnclosingVisualViewport(
   aLayoutViewport = aLayoutViewport.MoveInsideAndClamp(aScrollableRect);
 }
 
+void FrameMetrics::ApplyScrollUpdateFrom(const FrameMetrics& aContentMetrics) {
+  // In applying a main-thread scroll update, try to preserve the relative
+  // offset between the visual and layout viewports.
+  CSSPoint relativeOffset = GetVisualScrollOffset() - GetLayoutScrollOffset();
+  MOZ_ASSERT(IsRootContent() || relativeOffset == CSSPoint());
+  // We need to set the two offsets together, otherwise a subsequent
+  // RecalculateLayoutViewportOffset() could see divergent layout and
+  // visual offsets.
+  SetLayoutScrollOffset(aContentMetrics.GetLayoutScrollOffset());
+  ClampAndSetVisualScrollOffset(aContentMetrics.GetLayoutScrollOffset() +
+                                relativeOffset);
+
+  mScrollGeneration = aContentMetrics.mScrollGeneration;
+}
+
 ScrollSnapInfo::ScrollSnapInfo()
     : mScrollSnapStrictnessX(StyleScrollSnapStrictness::None),
       mScrollSnapStrictnessY(StyleScrollSnapStrictness::None) {}

gfx/layers/FrameMetrics.h

@@ -270,13 +270,7 @@ struct FrameMetrics {
            aContentFrameMetrics.GetVisualScrollOffset();
   }
 
-  void ApplyScrollUpdateFrom(const FrameMetrics& aOther) {
-    // Note that we set this (APZ) metrics' visual scroll offset to
-    // the incoming (Gecko) metrics' layout scroll offset.
-    // This will change in bug 1543485.
-    SetVisualScrollOffset(aOther.GetLayoutScrollOffset());
-    mScrollGeneration = aOther.mScrollGeneration;
-  }
+  void ApplyScrollUpdateFrom(const FrameMetrics& aContentMetrics);
 
   void ApplySmoothScrollUpdateFrom(const FrameMetrics& aOther) {
     mSmoothScrollOffset = aOther.mSmoothScrollOffset;

gfx/layers/apz/test/gtest/TestBasic.cpp

@@ -492,3 +492,32 @@ TEST_F(APZCBasicTester, ResumeInterruptedTouchDrag_Bug1592435) {
   TouchUp(apzc, touchPos, mcc->Time());
 }
 #endif
+
+TEST_F(APZCBasicTester, RelativeScrollOffset) {
+  // Set up initial conditions: zoomed in, layout offset at (100, 100),
+  // visual offset at (120, 120); the relative offset is such (20, 20).
+  ScrollMetadata metadata;
+  FrameMetrics& metrics = metadata.GetMetrics();
+  metrics.SetScrollableRect(CSSRect(0, 0, 1000, 1000));
+  metrics.SetLayoutViewport(CSSRect(100, 100, 100, 100));
+  metrics.SetZoom(CSSToParentLayerScale2D(2.0, 2.0));
+  metrics.SetCompositionBounds(ParentLayerRect(0, 0, 100, 100));
+  metrics.SetVisualScrollOffset(CSSPoint(120, 120));
+  metrics.SetIsRootContent(true);
+  apzc->SetFrameMetrics(metrics);
+
+  // Scroll the layout viewport to (200, 200).
+  ScrollMetadata mainThreadMetadata = metadata;
+  FrameMetrics& mainThreadMetrics = mainThreadMetadata.GetMetrics();
+  mainThreadMetrics.SetLayoutScrollOffset(CSSPoint(200, 200));
+  mainThreadMetrics.SetScrollOffsetUpdateType(FrameMetrics::eMainThread);
+  mainThreadMetrics.SetScrollGeneration(
+      mainThreadMetrics.GetScrollGeneration() + 1);
+  apzc->NotifyLayersUpdated(mainThreadMetadata, /*isFirstPaint=*/false,
+                            /*thisLayerTreeUpdated=*/true);
+
+  // Check that the relative offset has been preserved.
+  metrics = apzc->GetFrameMetrics();
+  EXPECT_EQ(metrics.GetLayoutScrollOffset(), CSSPoint(200, 200));
+  EXPECT_EQ(metrics.GetVisualScrollOffset(), CSSPoint(220, 220));
+}