`contain:content` breaks fullscreen

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Simon Fraser (smfr)]

Taking the YouTube embed fullscreen on https://www.fluentu.com/blog/french/funny-french-videos/ is broken with containment enabled; you see the page content, not the video in fullscreen.

CSS has:
lite-youtube {
    background-color: #000;
    position: relative;
    display: block;
    contain: content;
    background-position: center center;
    background-size: cover;
    cursor: pointer;
    max-width: 720px;
}
If I remove contain: content the bug does not occur.

This site seems to be using https://github.com/paulirish/lite-youtube-embed, so this might be common.

[Simon Fraser (smfr)]

<rdar://88689388>

[Simon Fraser (smfr)]

Created attachment 451615 [details]
Simple testcase

[zalan]

diff --git a/Source/WebCore/css/fullscreen.css b/Source/WebCore/css/fullscreen.css
index c2a71ca3ceda..b67ed6ee0d01 100644
--- a/Source/WebCore/css/fullscreen.css
+++ b/Source/WebCore/css/fullscreen.css
@@ -48,6 +48,7 @@
   perspective: none !important;
   -webkit-transform-style: flat !important;
   overflow: visible !important;
+  contain: none !important;
 }
 
 video:-webkit-full-screen, audio:-webkit-full-screen {
diff --git a/Source/WebCore/css/parser/CSSParserContext.cpp b/Source/WebCore/css/parser/CSSParserContext.cpp
index 55d36bb58616..f6b19755e2ea 100644
--- a/Source/WebCore/css/parser/CSSParserContext.cpp
+++ b/Source/WebCore/css/parser/CSSParserContext.cpp
@@ -51,6 +51,7 @@ CSSParserContext::CSSParserContext(CSSParserMode mode, const URL& baseURL)
     individualTransformPropertiesEnabled = true;
     focusVisibleEnabled = true;
     inputSecurityEnabled = true;
+    containmentEnabled = true;
 #if ENABLE(CSS_TRANSFORM_STYLE_OPTIMIZED_3D)
     transformStyleOptimized3DEnabled = true;
 #endif

will submit a patch once I figured out how to test fullscreen.

[zalan]

Created attachment 451710 [details]
Patch

[EWS]

Committed r289686 (247172@main): <https://commits.webkit.org/247172@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 451710 [details].