Bug 1358771 - Don't set the resolution on the main-thread from an APZ repaint request that was triggered by a main-thread update. r=tnikkel

Conceptually, a repaint request that arrives in APZCCallbackHelper with
!GetScrollOffsetUpdated() is a repaint request that the APZ is sending in
response to a main-thread update it received. The purpose of the repaint
request is mainly to update the displayport margins; the scroll position
and resolution should not have changed. Therefore actively trying to set
the scroll position or resolution as a result of these repaint requests
should at best be a no-op but often triggers unnecessary clobbering of things
that are in-flight, or extra reflows. This can result in buggy behaviour.
Bug 1286179 avoided doing this for the scroll position, and this patch does
the equivalent thing for zooming.

MozReview-Commit-ID: 91RbRP0ZHBe

--HG--
extra : rebase_source : 7bb97075619f5435705d369a695d37aa1c972278

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -251,11 +251,17 @@ APZCCallbackHelper::UpdateRootFrame(FrameMetrics& aMetrics)
 
   MOZ_ASSERT(aMetrics.GetUseDisplayPortMargins());
 
-  if (gfxPrefs::APZAllowZooming()) {
+  if (gfxPrefs::APZAllowZooming() && aMetrics.GetScrollOffsetUpdated()) {
     // If zooming is disabled then we don't really want to let APZ fiddle
     // with these things. In theory setting the resolution here should be a
     // no-op, but setting the SPCSPS is bad because it can cause a stale value
     // to be returned by window.innerWidth/innerHeight (see bug 1187792).
+    //
+    // We also skip this codepath unless the metrics has a scroll offset update
+    // type other eNone, because eNone just means that this repaint request
+    // was triggered by APZ in response to a main-thread update. In this
+    // scenario we don't want to update the main-thread resolution because
+    // it can trigger unnecessary reflows.
 
     float presShellResolution = shell->GetResolution();