Bug 1655238 - Only cancel existing APZ animations on mousedown when the mousedown lands on a scrollthumb. r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D85453

gfx/layers/apz/src/InputQueue.cpp

@@ -227,7 +227,9 @@ nsEventStatus InputQueue::ReceiveMouseInput(
 
     mActiveDragBlock = block;
 
-    CancelAnimationsForNewBlock(block);
+    if (aFlags.mHitScrollThumb) {
+      CancelAnimationsForNewBlock(block);
+    }
     MaybeRequestContentResponse(aTarget, block);
   }