Don't reuse input blocks with dead APZCs. (bug 1110038, r=kats)

--HG--
extra : rebase_source : 458816195b9e8e0394bfdf662b7173328015415c

gfx/layers/apz/src/InputQueue.cpp

@@ -127,7 +127,12 @@ InputQueue::ReceiveScrollWheelInput(const nsRefPtr<AsyncPanZoomController>& aTar
                                     uint64_t* aOutInputBlockId) {
   WheelBlockState* block = nullptr;
   if (!mInputBlockQueue.IsEmpty()) {
-    block = mInputBlockQueue.LastElement().get()->AsWheelBlock();
+    block = mInputBlockQueue.LastElement()->AsWheelBlock();
+
+    // If the block's APZC has been destroyed, request a new block.
+    if (block && block->GetTargetApzc()->IsDestroyed()) {
+      block = nullptr;
+    }
   }
 
   if (!block) {