Bug 1612750 - Simplify a misleading calculation in AsyncPanZoomController::GetCurrentAsyncTransform(). r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D62090

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

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4181,18 +4181,17 @@ AsyncTransform AsyncPanZoomController::GetCurrentAsyncTransform(
 
   ParentLayerPoint translation;
   if (aComponents.contains(AsyncTransformComponent::eVisual)) {
-    CSSPoint lastPaintVisualOffset;
+    // There is no "lastPaintVisualOffset" to subtract here; the visual offset
+    // is entirely async.
     if (mLastContentPaintMetrics.IsScrollable()) {
-      lastPaintVisualOffset =
-          mLastContentPaintMetrics.GetScrollOffset() -
-          mLastContentPaintMetrics.GetLayoutViewport().TopLeft();
+      MOZ_ASSERT(mLastContentPaintMetrics.GetScrollOffset() ==
+                 mLastContentPaintMetrics.GetLayoutViewport().TopLeft());
     }
 
     CSSPoint currentVisualOffset = GetEffectiveScrollOffset(aMode) -
                                    GetEffectiveLayoutViewport(aMode).TopLeft();
 
-    translation +=
-        (currentVisualOffset - lastPaintVisualOffset) * effectiveZoom;
+    translation += currentVisualOffset * effectiveZoom;
   }
   if (aComponents.contains(AsyncTransformComponent::eLayout)) {
     CSSPoint lastPaintLayoutOffset;

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

@@ -88,6 +88,7 @@ TEST_F(APZCBasicTester, ComplexTransform) {
   metrics.SetCompositionBounds(ParentLayerRect(0, 0, 24, 24));
   metrics.SetDisplayPort(CSSRect(-1, -1, 6, 6));
   metrics.SetScrollOffset(CSSPoint(10, 10));
+  metrics.SetLayoutViewport(CSSRect(10, 10, 8, 8));
   metrics.SetScrollableRect(CSSRect(0, 0, 50, 50));
   metrics.SetCumulativeResolution(LayoutDeviceToLayerScale2D(2, 2));
   metrics.SetPresShellResolution(2.0f);