Bug 1745529 - Use default action target to compute scroll amount. r=m…

…asayuki

This avoids pages having different scroll amount if there are overflow:
hidden elements with different font sizes.

Running through try to see if there's any test to adjust or I need to
write one.

Differential Revision: https://phabricator.services.mozilla.com/D133671

dom/events/EventStateManager.cpp

@@ -2847,7 +2847,12 @@ nsSize EventStateManager::GetScrollAmount(
   MOZ_ASSERT(aPresContext);
   MOZ_ASSERT(aEvent);
 
-  bool isPage = (aEvent->mDeltaMode == WheelEvent_Binding::DOM_DELTA_PAGE);
+  const bool isPage = aEvent->mDeltaMode == WheelEvent_Binding::DOM_DELTA_PAGE;
+  if (!aScrollableFrame) {
+    // If there is no scrollable frame, we should use root, see below.
+    aScrollableFrame = aPresContext->PresShell()->GetRootScrollFrameAsScrollable();
+  }
+
   if (aScrollableFrame) {
     return isPage ? aScrollableFrame->GetPageScrollAmount()
                   : aScrollableFrame->GetLineScrollAmount();
@@ -2858,7 +2863,12 @@ nsSize EventStateManager::GetScrollAmount(
     return aPresContext->GetVisibleArea().Size();
   }
 
-  // If there is no scrollable frame, we should use root frame's information.
+  // Otherwise use root frame's font metrics.
+  //
+  // FIXME(emilio): Should this use the root element's style frame? The root
+  // frame will always have the initial font. Then again it should never matter
+  // for content, we should always have a root scrollable frame in html
+  // documents.
   nsIFrame* rootFrame = aPresContext->PresShell()->GetRootFrame();
   if (!rootFrame) {
     return nsSize(0, 0);
@@ -6197,8 +6207,8 @@ void EventStateManager::DeltaAccumulator::InitLineOrPageDelta(
   mIsNoLineOrPageDeltaDevice = aEvent->mIsNoLineOrPageDelta;
 
   {
-    nsIFrame* frame = aESM->ComputeScrollTarget(
-        aTargetFrame, aEvent, COMPUTE_LEGACY_MOUSE_SCROLL_EVENT_TARGET);
+    nsIFrame* frame = aESM->ComputeScrollTarget(aTargetFrame, aEvent,
+                                                COMPUTE_DEFAULT_ACTION_TARGET);
     nsPresContext* pc =
         frame ? frame->PresContext() : aTargetFrame->PresContext();
     nsIScrollableFrame* scrollTarget = do_QueryFrame(frame);