Bug 1846575 - Scroll a full page per repeat timer tick during scrollbar track click-and-hold. r=rzvncj

Differential Revision: https://phabricator.services.mozilla.com/D187152

layout/xul/nsSliderFrame.cpp

@@ -1534,6 +1534,21 @@ void nsSliderFrame::PageScroll(bool aClickAndHold) {
 
     nsPoint pos = sf->GetScrollPosition();
 
+    if (mCurrentClickHoldDestination) {
+      // We may not have arrived at the destination of the scroll from the
+      // previous repeat timer tick, some of that scroll may still be pending.
+      nsPoint pendingScroll =
+          *mCurrentClickHoldDestination - sf->GetScrollPosition();
+
+      // Scroll by one page relative to the previous destination, so that we
+      // scroll at a rate of a full page per repeat timer tick.
+      pos += pendingScroll;
+
+      // Make a corresponding adjustment to the maxium distance we can scroll,
+      // so we successfully avoid overshoot.
+      maxDistanceToScroll -= (isHorizontal ? pendingScroll.x : pendingScroll.y);
+    }
+
     nscoord distanceToScroll =
         std::min(abs(maxDistanceToScroll),
                  CSSPixel::ToAppUnits(CSSCoord(pageLength))) *
@@ -1545,6 +1560,7 @@ void nsSliderFrame::PageScroll(bool aClickAndHold) {
       pos.y += distanceToScroll;
     }
 
+    mCurrentClickHoldDestination = Some(pos);
     sf->ScrollTo(pos,
                  StaticPrefs::general_smoothScroll() &&
                          StaticPrefs::general_smoothScroll_pages()

layout/xul/nsSliderFrame.h

@@ -184,7 +184,10 @@ class nsSliderFrame final : public nsContainerFrame {
     nsRepeatService::GetInstance()->Start(Notify, this, mContent->OwnerDoc(),
                                           "nsSliderFrame"_ns);
   }
-  void StopRepeat() { nsRepeatService::GetInstance()->Stop(Notify, this); }
+  void StopRepeat() {
+    nsRepeatService::GetInstance()->Stop(Notify, this);
+    mCurrentClickHoldDestination = Nothing();
+  }
   void Notify();
   static void Notify(void* aData) {
     (static_cast<nsSliderFrame*>(aData))->Notify();
@@ -192,6 +195,10 @@ class nsSliderFrame final : public nsContainerFrame {
   void PageScroll(bool aClickAndHold);
 
   nsPoint mDestinationPoint;
+  // If we are in a scrollbar track click-and-hold, this is populated with
+  // the destination of the scroll started at the most recent tick of the
+  // repeat timer.
+  Maybe<nsPoint> mCurrentClickHoldDestination;
   RefPtr<nsSliderMediator> mMediator;
 
   float mRatio;