From afc4a66d0c74204bef9dfa09907949e4e2d56e05 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emilio=20Cobos=20=C3=81lvarez?= <emilio@crisal.io>
Date: Wed, 4 Dec 2019 22:33:32 +0000
Subject: [PATCH] Bug 687441 - Make iframes not cause a FOUC on parent
 document. r=bzbarsky

This makes subframes not start layout on the parent document when flushing,
which fixes the FOUC issue.

It does mean that some resource loads triggered by viewport-dependent media
queries might not block onload of the frame, but other style-triggered loads,
fonts, and such would.

This makes same-origin frames act the same way as cross-origin iframes in this
regard if there are stylesheet loads present.

I think the trade-off is worth it here, and this is green on try.

Differential Revision: https://phabricator.services.mozilla.com/D55860

--HG--
extra : moz-landing-system : lando
---
 dom/base/Document.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index af80d21d4c41..d7b660531f91 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -9793,7 +9793,7 @@ void Document::FlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
   // layout flush on our parent, since we need our container to be the
   // correct size to determine the correct style.
   if (StyleOrLayoutObservablyDependsOnParentDocumentLayout() &&
-      IsSafeToFlush()) {
+      mParentDocument->MayStartLayout() && IsSafeToFlush()) {
     mozilla::ChangesToFlush parentFlush = aFlush;
     if (flushType >= FlushType::Style) {
       parentFlush.mFlushType = std::max(FlushType::Layout, flushType);