Backed out changeset ef564358a7e8 (bug 1778983) for causing wpt failu…

…res in CSSTransition-canceling.tentative.html CLOSED TREE

layout/base/PresShell.cpp

@@ -4333,6 +4333,29 @@ void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
       LAYOUT_TELEMETRY_RECORD_BASE(Restyle);
 
       mPresContext->RestyleManager()->ProcessPendingRestyles();
+    }
+
+    // Now those constructors or events might have posted restyle
+    // events.  At the same time, we still need up-to-date style data.
+    // In particular, reflow depends on style being completely up to
+    // date.  If it's not, then style reparenting, which can
+    // happen during reflow, might suddenly pick up the new rules and
+    // we'll end up with frames whose style doesn't match the frame
+    // type.
+    if (MOZ_LIKELY(!mIsDestroying)) {
+      nsAutoScriptBlocker scriptBlocker;
+      Maybe<uint64_t> innerWindowID;
+      if (auto* window = mDocument->GetInnerWindow()) {
+        innerWindowID = Some(window->WindowID());
+      }
+      AutoProfilerStyleMarker tracingStyleFlush(std::move(mStyleCause),
+                                                innerWindowID);
+      PerfStats::AutoMetricRecording<PerfStats::Metric::Styling> autoRecording;
+      LAYOUT_TELEMETRY_RECORD_BASE(Restyle);
+
+      mPresContext->RestyleManager()->ProcessPendingRestyles();
+      // Clear mNeedStyleFlush here agagin to make this flag work properly for
+      // optimization since the flag might have set in ProcessPendingRestyles().
       mNeedStyleFlush = false;
     }