From 7489a0349fa7eb18efeaa790ca09e81f8f30f72d Mon Sep 17 00:00:00 2001
From: Jo Steven Novaryo <65610990+stevennovaryo@users.noreply.github.com>
Date: Mon, 18 Aug 2025 19:25:31 +0800
Subject: [PATCH] layout: Do not include `position:fixed` children when
 calculating scrollable overflow for root element (#38618)

Reimplementation of: #35931

For a `FragmentTree` we define a scrollable overflow calculation that
includes the overflow all of it's children `Fragments`. In practice we
are using this calculation for scrolling area of the viewport and
defining the root scroll frames. However, since uncontained fixed
positioned element is located outside of the document and should not be
scrolled, and therefore it would make no sense to include them in the
calculation of its scrollable overflow as well.

Testing: New and existing WPT tests
Fixes: #38617
Fixes: #38182

---------

Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>
---
 .../layout/fragment_tree/fragment_tree.rs     | 24 +++++++++++++--
 tests/wpt/meta/MANIFEST.json                  |  7 +++++
 .../clip-path/clip-path-fixed-scroll.html.ini |  2 --
 ...d-positioned-initial-containing-block.html | 30 +++++++++++++++++++
 4 files changed, 58 insertions(+), 5 deletions(-)
 delete mode 100644 tests/wpt/meta/css/css-masking/clip-path/clip-path-fixed-scroll.html.ini
 create mode 100644 tests/wpt/tests/css/css-overflow/scrollable-overflow-fixed-positioned-initial-containing-block.html

diff --git a/components/layout/fragment_tree/fragment_tree.rs b/components/layout/fragment_tree/fragment_tree.rs
index 4b6e207fcc1..852e7f2e0bd 100644
--- a/components/layout/fragment_tree/fragment_tree.rs
+++ b/components/layout/fragment_tree/fragment_tree.rs
@@ -10,6 +10,7 @@ use compositing_traits::display_list::AxesScrollSensitivity;
 use fxhash::FxHashSet;
 use malloc_size_of_derive::MallocSizeOf;
 use style::animation::AnimationSetKey;
+use style::computed_values::position::T as Position;
 
 use super::{BoxFragment, ContainingBlockManager, Fragment};
 use crate::ArcRefCell;
@@ -124,9 +125,26 @@ impl FragmentTree {
             let scrollable_overflow = self.root_fragments.iter().fold(
                 self.initial_containing_block,
                 |overflow, fragment| {
-                    fragment
-                        .calculate_scrollable_overflow_for_parent()
-                        .union(&overflow)
+                    // Need to calculate the overflow for each fragments within the tree
+                    // because it is required in the next stages of reflow.
+                    let overflow_from_fragment =
+                        fragment.calculate_scrollable_overflow_for_parent();
+
+                    // Scrollable overflow should be accumulated in the block that
+                    // establishes the containing block for the element. Thus, fixed
+                    // positioned fragments whose containing block is the initial
+                    // containing block should not be included in overflow calculation.
+                    // See <https://www.w3.org/TR/css-overflow-3/#scrollable>.
+                    if fragment
+                        .retrieve_box_fragment()
+                        .is_some_and(|box_fragment| {
+                            box_fragment.borrow().style.get_box().position == Position::Fixed
+                        })
+                    {
+                        return overflow;
+                    }
+
+                    overflow.union(&overflow_from_fragment)
                 },
             );
 
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index 748f67ed36e..3bf5553a43a 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -607585,6 +607585,13 @@
        }
       ]
      ],
+     "scrollable-overflow-fixed-positioned-initial-containing-block.html": [
+      "10f115a012a85230a4641a1d793c0a6df0db2e78",
+      [
+       null,
+       {}
+      ]
+     ],
      "scrollable-overflow-float.html": [
       "f75c0a66cfffdfe9872c6e472966ee3cf639eae6",
       [
diff --git a/tests/wpt/meta/css/css-masking/clip-path/clip-path-fixed-scroll.html.ini b/tests/wpt/meta/css/css-masking/clip-path/clip-path-fixed-scroll.html.ini
deleted file mode 100644
index 7addfba3787..00000000000
--- a/tests/wpt/meta/css/css-masking/clip-path/clip-path-fixed-scroll.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[clip-path-fixed-scroll.html]
-  expected: FAIL
diff --git a/tests/wpt/tests/css/css-overflow/scrollable-overflow-fixed-positioned-initial-containing-block.html b/tests/wpt/tests/css/css-overflow/scrollable-overflow-fixed-positioned-initial-containing-block.html
new file mode 100644
index 00000000000..10f115a012a
--- /dev/null
+++ b/tests/wpt/tests/css/css-overflow/scrollable-overflow-fixed-positioned-initial-containing-block.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html>
+<head>
+<title>Scrollable overflow of Document element with 'position: fixed' which has ICB as containing block</title>
+<link rel="help" href="https://drafts.csswg.org/css-overflow/#scrollable">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#dom-element-scrollwidth">
+<link rel="help" href="https://drafts.csswg.org/cssom-view/#dom-element-scrollheight">
+<link rel="author" title="Jo Steven Novaryo" href="mailto:jo.steven.novaryo@huawei.com">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+</head>
+<body>
+</body>
+<script>
+promise_test(async (t) => {
+  let initialScrollHeight = document.scrollingElement.scrollHeight;
+  let initialScrollWidth = document.scrollingElement.scrollWidth;
+
+  let fixedDiv = document.createElement("div");
+  fixedDiv.style.position = "fixed";
+  fixedDiv.style.height = (initialScrollHeight * 2) + "px";
+  fixedDiv.style.width = (initialScrollWidth * 2) + "px";
+  document.body.appendChild(fixedDiv);
+
+  assert_equals(document.scrollingElement.scrollHeight, initialScrollHeight);
+  assert_equals(document.scrollingElement.scrollWidth, initialScrollWidth);
+
+}, "position: fixed fixed with ICB as CB should not affect scrollable overflow of Document element.");
+</script>
+</html>