Bug 1799425. Only set checkboard flag when adding background color item for root content document. r=emilio

Webrender uses this flag as a hint to separate mix blend containers so it should only be present on the root content document.

Differential Revision: https://phabricator.services.mozilla.com/D162046

layout/base/PresShell.cpp

@@ -5428,7 +5428,8 @@ void PresShell::AddCanvasBackgroundColorItem(
   if (!addedScrollingBackgroundColor || forceUnscrolledItem) {
     nsDisplaySolidColor* item = MakeDisplayItem<nsDisplaySolidColor>(
         aBuilder, aFrame, aBounds, bgcolor);
-    if (addedScrollingBackgroundColor) {
+    if (addedScrollingBackgroundColor &&
+        mPresContext->IsRootContentDocumentCrossProcess()) {
       item->SetIsCheckerboardBackground();
     }
     AddDisplayItemToBottom(aBuilder, aList, item);

layout/reftests/bugs/1799425-1-ref.html

@@ -0,0 +1,21 @@
+<style>
+body,
+html {
+  background-color: white;
+}
+.wrapper {
+  mix-blend-mode: difference;
+}
+.text {
+  color: white;
+}
+</style>
+<div class="wrapper">
+  <div class="text">This text should always be black
+  </div>
+</div>
+
+<div id="video">
+  <div>Hover here to make video disappear</div>
+  <iframe srcdoc="<div style='width: 100px; height: 100px; background: blue;'></div>">
+</div>

layout/reftests/bugs/1799425-1.html

@@ -0,0 +1,26 @@
+<style>
+body,
+html {
+  background-color: white;
+}
+.wrapper {
+  mix-blend-mode: difference;
+}
+.text {
+  color: white;
+}
+</style>
+<div class="wrapper">
+  <div class="text">This text should always be black
+  </div>
+</div>
+
+<div id="video">
+  <div>Hover here to make video disappear</div>
+  <iframe srcdoc="<style>
+	html, body {
+		background: white;
+	}
+  </style>
+<div style='width: 100px; height: 100px; background: blue;'></div>">
+</div>

layout/reftests/bugs/reftest.list

@@ -2132,4 +2132,5 @@ pref(layout.css.prefers-color-scheme.content-override,2) == 1787127.html 1787127
 == 1792643-1.html 1792643-1-ref.html
 skip-if(!/^Windows\x20NT\x2010\.0/.test(http.oscpu)) == 1798297-1.html 1798297-1-ref.html
 skip-if(!/^Windows\x20NT\x2010\.0/.test(http.oscpu)) != 1798297-1.html 1798297-1-notref.html
+== 1799425-1.html 1799425-1-ref.html
 == 1800437-1.html 1800437-1-ref.html

testing/web-platform/meta/css/CSS2/box-display/root-canvas-001.xht.ini

@@ -1,3 +0,0 @@
-[root-canvas-001.xht]
-  fuzzy:
-    if os == "win": maxDifference=91-93;totalPixels=2250-2300