Bug 1875011 - Use non-rounded scroll position behind layout.scroll.disable-pixel-alignment in nsHTMLScrollFrame::ScrollByCSSPixelsInternal. r=botond

Depends on D198766

Differential Revision: https://phabricator.services.mozilla.com/D198767

layout/generic/nsGfxScrollFrame.cpp

@@ -5006,7 +5006,12 @@ void nsHTMLScrollFrame::ScrollByCSSPixelsInternal(const CSSIntPoint& aDelta,
   // the previous scrolling operation, but there may be some edge cases where
   // the current position in CSS pixels differs from the given position, the
   // cases should be fixed in bug 1556685.
-  CSSIntPoint currentCSSPixels = GetRoundedScrollPositionCSSPixels();
+  CSSPoint currentCSSPixels;
+  if (StaticPrefs::layout_scroll_disable_pixel_alignment()) {
+    currentCSSPixels = GetScrollPositionCSSPixels();
+  } else {
+    currentCSSPixels = GetRoundedScrollPositionCSSPixels();
+  }
   nsPoint pt = CSSPoint::ToAppUnits(currentCSSPixels + aDelta);
 
   nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f);

layout/generic/nsGfxScrollFrame.h

@@ -79,6 +79,7 @@ class nsHTMLScrollFrame : public nsContainerFrame,
   using InScrollingGesture = nsIScrollableFrame::InScrollingGesture;
 
   using CSSIntPoint = mozilla::CSSIntPoint;
+  using CSSPoint = mozilla::CSSPoint;
   using ScrollReflowInput = mozilla::ScrollReflowInput;
   using ScrollAnchorContainer = mozilla::layout::ScrollAnchorContainer;
   friend nsHTMLScrollFrame* NS_NewHTMLScrollFrame(
@@ -702,6 +703,10 @@ class nsHTMLScrollFrame : public nsContainerFrame,
     mMayScheduleScrollAnimations = false;
   }
 
+  CSSPoint GetScrollPositionCSSPixels() const {
+    return CSSPoint::FromAppUnits(GetScrollPosition());
+  }
+
  public:
   void UpdateSticky();