Bugzilla

Had some time to try your suggestion today and wrote this very simple patch:
```diff
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -748,6 +748,7 @@ AsyncPanZoomController::AsyncPanZoomCont
       mPinchPaintTimerSet(false),
       mTestAttributeAppliers(0),
       mTestHasAsyncKeyScrolled(false),
+      mCurrentIsRootContent(false),
       mCheckerboardEventLock("APZCBELock") {
   if (aGestures == USE_GESTURE_DETECTOR) {
     mGestureEventListener = new GestureEventListener(this);
@@ -5159,7 +5160,12 @@ void AsyncPanZoomController::NotifyLayer
   bool viewportSizeUpdated = false;
   bool needToReclampScroll = false;
 
-  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault) {
+  bool isRootContent = Metrics().IsRootContent();
+  bool hasIsRootContentChanged = isRootContent != mCurrentIsRootContent;
+  mCurrentIsRootContent = isRootContent;
+
+  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault ||
+      hasIsRootContentChanged) {
     // Initialize our internal state to something sane when the content
     // that was just painted is something we knew nothing about previously
     CancelAnimation();
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.h b/gfx/layers/apz/src/AsyncPanZoomController.h
--- a/gfx/layers/apz/src/AsyncPanZoomController.h
+++ b/gfx/layers/apz/src/AsyncPanZoomController.h
@@ -1771,6 +1771,8 @@ class AsyncPanZoomController {
   uint8_t mTestAttributeAppliers;
   // Flag to track whether or not this APZC has ever async key scrolled.
   bool mTestHasAsyncKeyScrolled;
+  // Cache the last value of IsRootContent() seen by NotifyLayersUpdate().
+  bool mCurrentIsRootContent;
 
   /* ===================================================================
    * The functions and members in this section are used for checkerboard
```
Unfortunately, this trips another assert with the test HTML file provided:
```
Assertion failure: apzc->IsRootContent(), at /mnt/usb/work/debug/mozilla-unified/gfx/layers/apz/src/APZCTreeManager.cpp:758
```
Which is this code:
```c++
 756     if (Maybe<uint64_t> zoomAnimationId = apzc->GetZoomAnimationId()) {
 757       // for now we only support zooming on root content APZCs
 758       MOZ_ASSERT(apzc->IsRootContent());
 759 
 760       LayoutDeviceToParentLayerScale zoom = apzc->GetCurrentPinchZoomScale(
 761           AsyncPanZoomController::eForCompositing);
```
So either I've misunderstood your suggestion, or we appear to need a different strategy.
```

Had some time to try your suggestion today and wrote this very simple patch:
```diff
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -748,6 +748,7 @@ AsyncPanZoomController::AsyncPanZoomCont
       mPinchPaintTimerSet(false),
       mTestAttributeAppliers(0),
       mTestHasAsyncKeyScrolled(false),
+      mCurrentIsRootContent(false),
       mCheckerboardEventLock("APZCBELock") {
   if (aGestures == USE_GESTURE_DETECTOR) {
     mGestureEventListener = new GestureEventListener(this);
@@ -5159,7 +5160,12 @@ void AsyncPanZoomController::NotifyLayer
   bool viewportSizeUpdated = false;
   bool needToReclampScroll = false;
 
-  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault) {
+  bool isRootContent = Metrics().IsRootContent();
+  bool hasIsRootContentChanged = isRootContent != mCurrentIsRootContent;
+  mCurrentIsRootContent = isRootContent;
+
+  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault ||
+      hasIsRootContentChanged) {
     // Initialize our internal state to something sane when the content
     // that was just painted is something we knew nothing about previously
     CancelAnimation();
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.h b/gfx/layers/apz/src/AsyncPanZoomController.h
--- a/gfx/layers/apz/src/AsyncPanZoomController.h
+++ b/gfx/layers/apz/src/AsyncPanZoomController.h
@@ -1771,6 +1771,8 @@ class AsyncPanZoomController {
   uint8_t mTestAttributeAppliers;
   // Flag to track whether or not this APZC has ever async key scrolled.
   bool mTestHasAsyncKeyScrolled;
+  // Cache the last value of IsRootContent() seen by NotifyLayersUpdate().
+  bool mCurrentIsRootContent;
 
   /* ===================================================================
    * The functions and members in this section are used for checkerboard
```
Unfortunately, this trips another assert with the test HTML file provided:
```
Assertion failure: apzc->IsRootContent(), at /mnt/usb/work/debug/mozilla-unified/gfx/layers/apz/src/APZCTreeManager.cpp:758
```
Which is this code:
```c++
 756     if (Maybe<uint64_t> zoomAnimationId = apzc->GetZoomAnimationId()) {
 757       // for now we only support zooming on root content APZCs
 758       MOZ_ASSERT(apzc->IsRootContent());
 759 
 760       LayoutDeviceToParentLayerScale zoom = apzc->GetCurrentPinchZoomScale(
 761           AsyncPanZoomController::eForCompositing);
```
So either I've misunderstood your suggestion, or we appear to need a different strategy.