Bug 1675950 - Check for explicit restyles posted to pseudo-elements to check whether we need a style flush in getComputedStyle. r=heycam

Font loading invalidation may invalidate only pseudo-elements, like it
happens on this test-case.

Once we get to testing first-line / first-letter in the test-case
(css/css-values/ch-pseudo-recalc-on-font-load.html), we end up flushing
because those end up posting the restyle to the parent element
correctly.

Differential Revision: https://phabricator.services.mozilla.com/D96373

layout/style/nsComputedDOMStyle.cpp

@@ -83,6 +83,22 @@ static nsDOMCSSValueList* GetROCSSValueList(bool aCommaDelimited) {
   return new nsDOMCSSValueList(aCommaDelimited);
 }
 
+static Element* GetRenderedElement(Element* aElement, nsAtom* aPseudo) {
+  if (aPseudo == nsCSSPseudoElements::before()) {
+    return nsLayoutUtils::GetBeforePseudo(aElement);
+  }
+  if (aPseudo == nsCSSPseudoElements::after()) {
+    return nsLayoutUtils::GetAfterPseudo(aElement);
+  }
+  if (aPseudo == nsCSSPseudoElements::marker()) {
+    return nsLayoutUtils::GetMarkerPseudo(aElement);
+  }
+  if (!aPseudo) {
+    return aElement;
+  }
+  return nullptr;
+}
+
 // Whether aDocument needs to restyle for aElement
 static bool ElementNeedsRestyle(Element* aElement, nsAtom* aPseudo,
                                 bool aMayNeedToFlushLayout) {
@@ -151,12 +167,12 @@ static bool ElementNeedsRestyle(Element* aElement, nsAtom* aPseudo,
     return false;
   }
 
-  // Then if there is a restyle root, we check if the root is an ancestor of
-  // this content. If it is not, then we don't need to restyle immediately.
-  // Note this is different from Gecko: we only check if any ancestor needs
-  // to restyle _itself_, not descendants, since dirty descendants can be
-  // another subtree.
-  return Servo_HasPendingRestyleAncestor(aElement, aMayNeedToFlushLayout);
+  // If there's a pseudo, we need to prefer that element, as the pseudo itself
+  // may have explicit restyles.
+  Element* styledElement = GetRenderedElement(aElement, aPseudo);
+  // Try to skip the restyle otherwise.
+  return Servo_HasPendingRestyleAncestor(
+      styledElement ? styledElement : aElement, aMayNeedToFlushLayout);
 }
 
 /**
@@ -516,18 +532,7 @@ already_AddRefed<ComputedStyle> nsComputedDOMStyle::DoGetComputedStyleNoFlush(
   // mPrimaryFrame). Remove it once that's fixed.
   if (inDocWithShell && aStyleType == eAll &&
       !aElement->IsHTMLElement(nsGkAtoms::area)) {
-    Element* element = nullptr;
-    if (aPseudo == nsCSSPseudoElements::before()) {
-      element = nsLayoutUtils::GetBeforePseudo(aElement);
-    } else if (aPseudo == nsCSSPseudoElements::after()) {
-      element = nsLayoutUtils::GetAfterPseudo(aElement);
-    } else if (aPseudo == nsCSSPseudoElements::marker()) {
-      element = nsLayoutUtils::GetMarkerPseudo(aElement);
-    } else if (!aPseudo) {
-      element = aElement;
-    }
-
-    if (element) {
+    if (Element* element = GetRenderedElement(aElement, aPseudo)) {
       if (nsIFrame* styleFrame = nsLayoutUtils::GetStyleFrame(element)) {
         ComputedStyle* result = styleFrame->Style();
         // Don't use the style if it was influenced by pseudo-elements,