[CSS Tables] scrollWidth incorrectly inflated by collapsed border overflow

Ahmad-S792 · Ahmad Saleem · commit 8f695151e326 · 2026-02-26T16:33:33.000-08:00
https://bugs.webkit.org/show_bug.cgi?id=291514 rdar://149675907 Reviewed by Alan Baradlay. This patch aligns WebKit with Gecko / Firefox and Blink / Chromium. Collapsed borders that extend outside a table's border box are a painting artifact and should not contribute to scrollable overflow. Previously, addOverflowFromInFlowChildren() passed the collapsed border overflow rect to both addLayoutOverflow() and addVisualOverflow(), causing the scroll container's scrollWidth/scrollHeight to include the outer half of any collapsed section/cell border. Remove the addLayoutOverflow() call so only visual overflow is recorded for collapsed border geometry. * LayoutTests/imported/w3c/web-platform-tests/css/css-tables/collapsed-scroll-overflow-expected.txt: * Source/WebCore/rendering/RenderTable.cpp: (WebCore::RenderTable::addOverflowFromInFlowChildren): Canonical link: https://commits.webkit.org/308311@main
diff --git a/LayoutTests/imported/w3c/web-platform-tests/css/css-tables/collapsed-scroll-overflow-expected.txt b/LayoutTests/imported/w3c/web-platform-tests/css/css-tables/collapsed-scroll-overflow-expected.txt
@@ -1,4 +1,4 @@
 header	body body body body body body body body body body body
 
-FAIL collapsed-scroll-overflow assert_equals: The collapsed border does not exceed the scroll container. expected 500 but got 540
+PASS collapsed-scroll-overflow
 
diff --git a/Source/WebCore/rendering/RenderTable.cpp b/Source/WebCore/rendering/RenderTable.cpp
@@ -4,7 +4,7 @@
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
- * Copyright (C) 2003-2025 Apple Inc. All rights reserved.
+ * Copyright (C) 2003-2026 Apple Inc. All rights reserved.
  * Copyright (C) 2014-2019 Google Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
@@ -810,7 +810,10 @@ void RenderTable::addOverflowFromInFlowChildren(OptionSet<ComputeOverflowOptions
         LayoutUnit topBorderOverflow = borderTop() - outerBorderTop();
         LayoutRect borderOverflowRect(leftBorderOverflow, topBorderOverflow, rightBorderOverflow - leftBorderOverflow, bottomBorderOverflow - topBorderOverflow);
         if (borderOverflowRect != borderBoxRect()) {
-            addLayoutOverflow(borderOverflowRect);
+            // Do NOT add layout overflow for collapsed borders — they are a
+            // painting artifact and must not inflate the scroll container's
+            // scrollWidth/scrollHeight.
+            // See https://github.com/w3c/csswg-drafts/issues/6230
             addVisualOverflow(borderOverflowRect);
         }
     }

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`	`1`	`header body body body body body body body body body body body`
`2`	`2`
`3`		`-FAIL collapsed-scroll-overflow assert_equals: The collapsed border does not exceed the scroll container. expected 500 but got 540`
	`3`	`+PASS collapsed-scroll-overflow`
`4`	`4`