Bug 1655130. Handle non-smooth scrolling with desktop zooming scrollbars. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D89408

gfx/layers/FrameMetrics.cpp

@@ -114,6 +114,14 @@ CSSPoint FrameMetrics::ApplyRelativeScrollUpdateFrom(
   return GetVisualScrollOffset() - origin;
 }
 
+CSSPoint FrameMetrics::ApplyPureRelativeScrollUpdateFrom(
+    const ScrollPositionUpdate& aUpdate) {
+  MOZ_ASSERT(aUpdate.GetType() == ScrollUpdateType::PureRelative);
+  CSSPoint origin = GetVisualScrollOffset();
+  ClampAndSetVisualScrollOffset(origin + aUpdate.GetDelta());
+  return GetVisualScrollOffset() - origin;
+}
+
 ScrollSnapInfo::ScrollSnapInfo()
     : mScrollSnapStrictnessX(StyleScrollSnapStrictness::None),
       mScrollSnapStrictnessY(StyleScrollSnapStrictness::None) {}

gfx/layers/FrameMetrics.h

@@ -264,6 +264,9 @@ struct FrameMetrics {
    */
   CSSPoint ApplyRelativeScrollUpdateFrom(const ScrollPositionUpdate& aUpdate);
 
+  CSSPoint ApplyPureRelativeScrollUpdateFrom(
+      const ScrollPositionUpdate& aUpdate);
+
   void UpdatePendingScrollInfo(const ScrollPositionUpdate& aInfo) {
     SetLayoutScrollOffset(aInfo.GetDestination());
     mScrollGeneration = aInfo.GetGeneration();

gfx/layers/apz/src/APZPublicUtils.cpp

@@ -82,7 +82,7 @@ ComputeBezierAnimationSettingsForOrigin(ScrollOrigin aOrigin) {
   bool isOriginSmoothnessEnabled = false;
 
 #define READ_DURATIONS(prefbase) \
-  isOriginSmoothnessEnabled = StaticPrefs::general_smoothScroll_ ## prefbase (); \
+  isOriginSmoothnessEnabled = StaticPrefs::general_smoothScroll() && StaticPrefs::general_smoothScroll_ ## prefbase (); \
   if (isOriginSmoothnessEnabled) { \
     minMS = StaticPrefs::general_smoothScroll_ ## prefbase ## _durationMinMS(); \
     maxMS = StaticPrefs::general_smoothScroll_ ## prefbase ## _durationMaxMS(); \

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4697,6 +4697,30 @@ void AsyncPanZoomController::NotifyLayersUpdated(
       relativeDelta =
           Some(Metrics().ApplyRelativeScrollUpdateFrom(scrollUpdate));
       Metrics().RecalculateLayoutViewportOffset();
+    } else if (scrollUpdate.GetType() == ScrollUpdateType::PureRelative) {
+      APZC_LOG("%p pure-relative updating scroll offset from %s by %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDelta()).c_str());
+
+      // Always need a repaint request with a repaint type for pure relative
+      // scrolls because apz is doing the scroll at the main thread's request.
+      // The main thread has not updated it's scroll offset yet, it is depending
+      // on apz to tell it where to scroll.
+      needContentRepaint = true;
+      contentRepaintType = RepaintUpdateType::eVisualUpdate;
+
+      // We have to cancel a visual scroll offset update otherwise it will
+      // clobber the relative scrolling we are about to do. We perform
+      // visualScrollOffset = visualScrollOffset + delta. Then the
+      // visualScrollOffsetUpdated block below will do visualScrollOffset =
+      // aLayerMetrics.GetVisualDestination(). We need visual scroll offset
+      // updates to be incorporated into this scroll update loop to properly fix
+      // this.
+      visualScrollOffsetUpdated = false;
+
+      relativeDelta =
+          Some(Metrics().ApplyPureRelativeScrollUpdateFrom(scrollUpdate));
+      Metrics().RecalculateLayoutViewportOffset();
     } else {
       APZC_LOG("%p updating scroll offset from %s to %s\n", this,
                ToString(Metrics().GetVisualScrollOffset()).c_str(),