Make dialog::backdrop span the viewport by default.

Source/core/rendering/RenderTheme.cpp

Index: Source/core/rendering/RenderTheme.cpp
diff --git a/Source/core/rendering/RenderTheme.cpp b/Source/core/rendering/RenderTheme.cpp
index d0a7b19d71d081cf94f12ac1291e6c24b92eb5e1..87c69d0ba79ade89fab98c634b8d8f862c44bb2e 100644
--- a/Source/core/rendering/RenderTheme.cpp
+++ b/Source/core/rendering/RenderTheme.cpp
@@ -497,7 +497,7 @@ String RenderTheme::extraDefaultStyleSheet()
     if (RuntimeEnabledFeatures::dialogElementEnabled()) {
         runtimeCSS.appendLiteral("dialog:not([open]) { display: none; }");
         runtimeCSS.appendLiteral("dialog { position: absolute; left: 0; right: 0; width: -webkit-fit-content; height: -webkit-fit-content; margin: auto; border: solid; padding: 1em; background: white; color: black;}");
-        runtimeCSS.appendLiteral("dialog::backdrop { background: rgba(0,0,0,0.1); }");
+        runtimeCSS.appendLiteral("dialog::backdrop { position: fixed; top: 0; right: 0; bottom: 0; left: 0; background: rgba(0,0,0,0.1); }");
     }
 
     return runtimeCSS.toString();