Bug 1543485 - Back out the hacky fix for bug 1549625. r=kats

Now that main thread scroll updates preserve the relative scroll offset
in APZ, this hacky fix is no longer necessary.

Differential Revision: https://phabricator.services.mozilla.com/D87687

layout/generic/nsGfxScrollFrame.cpp

@@ -2926,23 +2926,6 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
       FrameLayerBuilder::GetPaintedLayerScaleForFrame(mScrolledFrame);
   nsPoint curPos = GetScrollPosition();
 
-  // Below, we clamp |aPt| to the layout scroll range, compare the clamped
-  // value with the current scroll position, and early exit if they are the
-  // same. The early exit bypasses the update of |mLastScrollOrigin|, which
-  // is important to avoid sending a main-thread layout scroll update to APZ,
-  // which can clobber the visual scroll offset as well.
-  // If the layout viewport has shrunk in size since the last scroll, the
-  // clamped target position can be different from the current position, so
-  // we don't take the early exit, but conceptually we still want to avoid
-  // updating |mLastScrollOrigin| and clobbering the visual scroll offset.
-  // We only do this if the visual and layout scroll offsets can diverge
-  // in the first place, because it causes various regressions (bug 1543485
-  // tracks a proper fix).
-  bool suppressScrollOriginChange = false;
-  if (StaticPrefs::apz_allow_zooming() && aPt == curPos) {
-    suppressScrollOriginChange = true;
-  }
-
   nsPoint alignWithPos = mScrollPosForLayerPixelAlignment == nsPoint(-1, -1)
                              ? curPos
                              : mScrollPosForLayerPixelAlignment;
@@ -3026,8 +3009,7 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
   // legitimate scroll offset updates because the origin has been masked by
   // a later change within the same refresh driver tick.
   allowScrollOriginChange =
-      (mAllowScrollOriginDowngrade || !isScrollOriginDowngrade) &&
-      !suppressScrollOriginChange;
+      (mAllowScrollOriginDowngrade || !isScrollOriginDowngrade);
 
   if (allowScrollOriginChange) {
     mLastScrollOrigin = aOrigin;