Bug 1575138 - Do not schedule reconstruction for <slot> if there's no fallback. r=smaug

Just realized that we probably want this too.

Differential Revision: https://phabricator.services.mozilla.com/D46898

--HG--
extra : moz-landing-system : lando

dom/base/ShadowRoot.cpp

@@ -638,7 +638,8 @@ void ShadowRoot::MaybeSlotHostChild(nsIContent& aChild) {
   }
 
   // Fallback content will go away, let layout know.
-  if (assignment.mSlot->AssignedNodes().IsEmpty()) {
+  if (assignment.mSlot->AssignedNodes().IsEmpty() &&
+      assignment.mSlot->HasChildren()) {
     InvalidateStyleAndLayoutOnSubtree(assignment.mSlot);
   }
 

layout/base/PresShell.cpp

@@ -2860,7 +2860,7 @@ void PresShell::SlotAssignmentWillChange(Element& aElement,
   if (aNewSlot) {
     // If the new slot will stop showing fallback content, we need to reframe it
     // altogether.
-    if (aNewSlot->AssignedNodes().IsEmpty()) {
+    if (aNewSlot->AssignedNodes().IsEmpty() && aNewSlot->HasChildren()) {
       DestroyFramesForAndRestyle(aNewSlot);
       // Otherwise we just care about the element, but we need to ensure that
       // something takes care of traversing to the relevant slot, if needed.