Bug 1866738 - Make initial about:blank docs always transparent, even …

…in content docshells. r=jwatt This implements the proposed fix to w3c/csswg-drafts#9624, and adds a tentative test for it. Differential Revision: https://phabricator.services.mozilla.com/D194751
mozilla · Nov 29, 2023 · d407a74 · d407a74
1 parent b261dd7
commit d407a74
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Showing 5 changed files with 59 additions and 7 deletions.
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
@@ -5359,16 +5359,35 @@ void PresShell::AddCanvasBackgroundColorItem(nsDisplayListBuilder* aBuilder,
 }
 
 bool PresShell::IsTransparentContainerElement() const {
+  if (mDocument->IsInitialDocument()) {
+    switch (StaticPrefs::layout_css_initial_document_transparency()) {
+      case 3:
+        return true;
+      case 2:
+        if (!mDocument->IsTopLevelContentDocument()) {
+          return true;
+        }
+        [[fallthrough]];
+      case 1:
+        if (mDocument->IsLikelyContentInaccessibleTopLevelAboutBlank()) {
+          return true;
+        }
+        [[fallthrough]];
+      default:
+        break;
+    }
+  }
+
   nsPresContext* pc = GetPresContext();
   if (!pc->IsRootContentDocumentCrossProcess()) {
-    if (pc->IsChrome()) {
+    if (mDocument->IsInChromeDocShell()) {
       return true;
     }
     // Frames are transparent except if their used embedder color-scheme is
     // mismatched, in which case we use an opaque background to avoid
     // black-on-black or white-on-white text, see
     // https://github.com/w3c/csswg-drafts/issues/4772
-    if (BrowsingContext* bc = pc->Document()->GetBrowsingContext()) {
+    if (BrowsingContext* bc = mDocument->GetBrowsingContext()) {
       switch (bc->GetEmbedderColorSchemes().mUsed) {
         case dom::PrefersColorSchemeOverride::Light:
           return pc->DefaultBackgroundColorScheme() == ColorScheme::Light;
@@ -5382,11 +5401,6 @@ bool PresShell::IsTransparentContainerElement() const {
     return true;
   }
 
-  if (mDocument->IsInitialDocument() &&
-      mDocument->IsLikelyContentInaccessibleTopLevelAboutBlank()) {
-    return true;
-  }
-
   nsIDocShell* docShell = pc->GetDocShell();
   if (!docShell) {
     return false;

diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
@@ -8221,6 +8221,22 @@
   value: true
   mirror: always
 
+# Controls the transparency of the initial about:blank document. Generally you
+# don't ever want a white flash in dark mode, but due to backwards compat we
+# have some extra control over this, for now at least.
+#
+# See https://github.com/w3c/csswg-drafts/issues/9624 for iframes.
+#
+# Values:
+#  1: content-inaccessible top-level only.
+#  2: frames and content-inaccessible top-level only.
+#  3: always
+#  Others: don't treat this document specially.
+- name: layout.css.initial-document-transparency
+  type: RelaxedAtomicInt32
+  value: 3
+  mirror: always
+
 # The minimum contrast ratio between the accent color foreground and background
 # colors.
 #

diff --git a/...or-adjust/rendering/dark-color-scheme/color-scheme-iframe-background-about-blank-ref.html b/...or-adjust/rendering/dark-color-scheme/color-scheme-iframe-background-about-blank-ref.html
@@ -0,0 +1,5 @@
+<!doctype html>
+<title>CSS Test Reference</title>
+<style>:root { color-scheme: dark }</style>
+<p>Should not see a white frame below</p>
+<iframe width="600" height="400" src="support/dark-frame-blank.html"></iframe>
diff --git a/...ust/rendering/dark-color-scheme/color-scheme-iframe-background-about-blank.tentative.html b/...ust/rendering/dark-color-scheme/color-scheme-iframe-background-about-blank.tentative.html
@@ -0,0 +1,13 @@
+<!doctype html>
+<title>CSS Color Adjustment Test: about:blank doesn't cause a white backdrop in dark mode even though technically its color-scheme is mismatched</title>
+<link rel="help" href="https://drafts.csswg.org/css-color-adjust/#color-scheme-effect">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/9624">
+<link rel="match" href="color-scheme-iframe-background-ref.html">
+<style>
+:root { color-scheme: dark }
+</style>
+<p>Should not see a white frame below</p>
+<iframe width="600" height="400"></iframe>
+<script>
+  document.querySelector("iframe").contentWindow.stop();
+</script>
diff --git a/...form/tests/css/css-color-adjust/rendering/dark-color-scheme/support/dark-frame-blank.html b/...form/tests/css/css-color-adjust/rendering/dark-color-scheme/support/dark-frame-blank.html
@@ -0,0 +1,4 @@
+<!doctype html>
+<style>
+  :root { color-scheme: dark }
+</style>