From 0de0f741b62b6cb04e3dfa8fd1cf7e263f145108 Mon Sep 17 00:00:00 2001
From: Oriol Brufau <obrufau@igalia.com>
Date: Fri, 19 Sep 2025 22:17:56 +0200
Subject: [PATCH] layout: Ignore insets in Taffy for static and sticky
 positionings (#39401)

Taffy treats static and sticky positionings as relative. So the inset
properties shifted the element in the relpos way, which was no good.
It's better to just treat them as `auto` instead.

Testing: 3 existing tests pass, and adding a new one.
Fixes: #39399

Signed-off-by: Oriol Brufau <obrufau@igalia.com>
---
 .../layout/taffy/stylo_taffy/wrapper.rs       | 14 ++++++++++
 tests/wpt/meta/MANIFEST.json                  | 13 +++++++++
 ...nment-sticky-positioned-items-002.html.ini |  6 -----
 ...nment-sticky-positioned-items-002.html.ini |  6 -----
 .../sticky/position-sticky-grid.html.ini      |  2 --
 .../css/css-position/position-static-001.html | 27 +++++++++++++++++++
 6 files changed, 54 insertions(+), 14 deletions(-)
 delete mode 100644 tests/wpt/meta/css/css-grid/alignment/grid-column-axis-alignment-sticky-positioned-items-002.html.ini
 delete mode 100644 tests/wpt/meta/css/css-grid/alignment/grid-row-axis-alignment-sticky-positioned-items-002.html.ini
 delete mode 100644 tests/wpt/meta/css/css-position/sticky/position-sticky-grid.html.ini
 create mode 100644 tests/wpt/tests/css/css-position/position-static-001.html

diff --git a/components/layout/taffy/stylo_taffy/wrapper.rs b/components/layout/taffy/stylo_taffy/wrapper.rs
index 01410c7c074..a0fe4fb6e63 100644
--- a/components/layout/taffy/stylo_taffy/wrapper.rs
+++ b/components/layout/taffy/stylo_taffy/wrapper.rs
@@ -10,6 +10,7 @@ use style::values::computed::{GridTemplateAreas, LengthPercentage};
 use style::values::generics::grid::{TrackListValue, TrackRepeat, TrackSize};
 use style::values::specified::position::NamedArea;
 use style::{Atom, OwnedSlice};
+use taffy::prelude::TaffyAuto;
 
 use super::{convert, stylo};
 
@@ -73,6 +74,19 @@ impl<T: Deref<Target = ComputedValues>> taffy::CoreStyle for TaffyStyloStyle<T>
 
     #[inline]
     fn inset(&self) -> taffy::Rect<taffy::LengthPercentageAuto> {
+        // Taffy doesn't support static nor sticky positionings, they are treated
+        // as relative. As a workaround, ignore the insets.
+        if matches!(
+            self.style.get_box().position,
+            stylo::Position::Static | stylo::Position::Sticky
+        ) {
+            return taffy::Rect {
+                left: taffy::LengthPercentageAuto::AUTO,
+                right: taffy::LengthPercentageAuto::AUTO,
+                top: taffy::LengthPercentageAuto::AUTO,
+                bottom: taffy::LengthPercentageAuto::AUTO,
+            };
+        }
         let position_styles = self.style.get_position();
         taffy::Rect {
             left: convert::inset(&position_styles.left),
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index e65ff27179f..7903ca7500f 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -237547,6 +237547,19 @@
        {}
       ]
      ],
+     "position-static-001.html": [
+      "38647cbb0feb64a50cf176379cbdcd8c4f189812",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-200px-square.html",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "replaced-object-backdrop.html": [
       "b9d2cb599bd4610aaa9161fbf7fd7f8a35e89b8b",
       [
diff --git a/tests/wpt/meta/css/css-grid/alignment/grid-column-axis-alignment-sticky-positioned-items-002.html.ini b/tests/wpt/meta/css/css-grid/alignment/grid-column-axis-alignment-sticky-positioned-items-002.html.ini
deleted file mode 100644
index 10eb51fdf1f..00000000000
--- a/tests/wpt/meta/css/css-grid/alignment/grid-column-axis-alignment-sticky-positioned-items-002.html.ini
+++ /dev/null
@@ -1,6 +0,0 @@
-[grid-column-axis-alignment-sticky-positioned-items-002.html]
-  [.grid 1]
-    expected: FAIL
-
-  [.grid 2]
-    expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/alignment/grid-row-axis-alignment-sticky-positioned-items-002.html.ini b/tests/wpt/meta/css/css-grid/alignment/grid-row-axis-alignment-sticky-positioned-items-002.html.ini
deleted file mode 100644
index 85e518e5b54..00000000000
--- a/tests/wpt/meta/css/css-grid/alignment/grid-row-axis-alignment-sticky-positioned-items-002.html.ini
+++ /dev/null
@@ -1,6 +0,0 @@
-[grid-row-axis-alignment-sticky-positioned-items-002.html]
-  [.grid 1]
-    expected: FAIL
-
-  [.grid 2]
-    expected: FAIL
diff --git a/tests/wpt/meta/css/css-position/sticky/position-sticky-grid.html.ini b/tests/wpt/meta/css/css-position/sticky/position-sticky-grid.html.ini
deleted file mode 100644
index 9a86e2d8c1f..00000000000
--- a/tests/wpt/meta/css/css-position/sticky/position-sticky-grid.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[position-sticky-grid.html]
-  expected: FAIL
diff --git a/tests/wpt/tests/css/css-position/position-static-001.html b/tests/wpt/tests/css/css-position/position-static-001.html
new file mode 100644
index 00000000000..38647cbb0fe
--- /dev/null
+++ b/tests/wpt/tests/css/css-position/position-static-001.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/css-position/#insets">
+<link rel="help" href="https://github.com/servo/servo/issues/39399">
+<link rel="match" href="../reference/ref-filled-green-200px-square.html">
+<meta name="assert" content="Inset propeties do not apply to a statically positioned grid item.">
+
+<style>
+#grid {
+  display: grid;
+  width: 200px;
+  height: 200px;
+  background: red;
+}
+#item {
+  width: 200px;
+  height: 200px;
+  top: 200px;
+  left: 200px;
+  background: green;
+}
+</style>
+
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div id="grid">
+  <div id="item"></div>
+</div>