Improve logic for establishing a stacking context (#35947)

In particular:
 - `z-index` will now work on unpositioned grid items.
 - `will-change: z-index` will only establish a stacking context if
   `z-index` applies, i.e. if the box is positioned or a flex/grid item.
 - The conditions in `establishes_stacking_context()` are reordered,
   so that the most likely ones are checked first.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

29 files changed, 127 insertions, 48 deletions

diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index 79668fb0ad6..e54ddc40d22 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -307667,6 +307667,45 @@
        {}
       ]
      ],
+     "will-change-stacking-context-z-index-2.html": [
+      "9379185048a78b031e8ac290bd7bb3e5df24fa5f",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "will-change-stacking-context-z-index-3.html": [
+      "4ec40ae1a430a095ea6f703c8c27698e04792220",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
+     "will-change-stacking-context-z-index-4.html": [
+      "d8a87b23415c3c10a6f4875f16cd63301e789152",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-100px-square.xht",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "will-change-transform-add-content.html": [
       "1d8568ee629e48adfe99abab2e2194a73ae1301a",
       [
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-001.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-001.html.ini
deleted file mode 100644
index 45cf6fc0460..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-002.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-002.html.ini
deleted file mode 100644
index 4ff1ef0fe2d..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-002.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-003.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-003.html.ini
deleted file mode 100644
index 837fbd2a8c1..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-003.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-004.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-004.html.ini
deleted file mode 100644
index fd41d4c1a42..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-004.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-005.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-005.html.ini
deleted file mode 100644
index 293ed0639f2..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-005.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-005.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-001.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-001.html.ini
deleted file mode 100644
index eb2233d3700..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-002.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-002.html.ini
deleted file mode 100644
index cb9905e7a27..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-002.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-003.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-003.html.ini
deleted file mode 100644
index f3a786abf7b..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-003.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-004.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-004.html.ini
deleted file mode 100644
index 03f883fbceb..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-004.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-005.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-005.html.ini
deleted file mode 100644
index f89f4e9cfbe..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-005.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-005.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-006.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-006.html.ini
deleted file mode 100644
index 479e338b1fd..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-inline-z-axis-ordering-overlapped-items-006.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-inline-z-axis-ordering-overlapped-items-006.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-a.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-a.html.ini
deleted file mode 100644
index 64b4b2b480e..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-a.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-layout-z-order-a.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-b.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-b.html.ini
deleted file mode 100644
index 7573aba43fd..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-layout-z-order-b.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-layout-z-order-b.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-001.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-001.html.ini
deleted file mode 100644
index 3a9b399710f..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-002.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-002.html.ini
deleted file mode 100644
index 3702bfaad3d..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-002.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-003.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-003.html.ini
deleted file mode 100644
index 0a3c12ac1bf..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-003.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-004.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-004.html.ini
deleted file mode 100644
index 221e23ddb98..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-004.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-005.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-005.html.ini
deleted file mode 100644
index 43014f76ca9..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-005.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-005.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-001.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-001.html.ini
deleted file mode 100644
index 98556a4a8ca..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-001.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-001.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-002.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-002.html.ini
deleted file mode 100644
index c947cd0caf8..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-002.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-002.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-003.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-003.html.ini
deleted file mode 100644
index 80644b4df38..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-003.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-003.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-004.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-004.html.ini
deleted file mode 100644
index c6498b7b9c0..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-004.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-004.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-005.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-005.html.ini
deleted file mode 100644
index c214befe848..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-005.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-005.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-006.html.ini b/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-006.html.ini
deleted file mode 100644
index b4f85a68692..00000000000
--- a/tests/wpt/meta/css/css-grid/grid-items/grid-z-axis-ordering-overlapped-items-006.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[grid-z-axis-ordering-overlapped-items-006.html]
-  expected: FAIL
diff --git a/tests/wpt/meta/css/css-will-change/will-change-stacking-context-z-index-3.html.ini b/tests/wpt/meta/css/css-will-change/will-change-stacking-context-z-index-3.html.ini
new file mode 100644
index 00000000000..d0c021b9dd0
--- /dev/null
+++ b/tests/wpt/meta/css/css-will-change/will-change-stacking-context-z-index-3.html.ini
@@ -0,0 +1,2 @@
+[will-change-stacking-context-z-index-3.html]
+  prefs: ["layout_grid_enabled:true"]
diff --git a/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-2.html b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-2.html
new file mode 100644
index 00000000000..9379185048a
--- /dev/null
+++ b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-2.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<title>CSS Test: `will-change: z-index`</title>
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://www.w3.org/TR/css-will-change/#valdef-will-change-custom-ident">
+<link rel="help" href="https://www.w3.org/TR/css-flexbox-1/#painting">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/11827">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  `will-change: z-index` establishes a stacking context on a flex item.
+">
+<style>
+.test {
+  will-change: z-index;
+  width: 100px;
+  background: red;
+}
+.test::before {
+  content: "";
+  display: block;
+  position: relative;
+  z-index: -1;
+  height: 100px;
+  background: green;
+}
+</style>
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="display: flex">
+  <div class="test"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-3.html b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-3.html
new file mode 100644
index 00000000000..4ec40ae1a43
--- /dev/null
+++ b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-3.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<title>CSS Test: `will-change: z-index`</title>
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://www.w3.org/TR/css-will-change/#valdef-will-change-custom-ident">
+<link rel="help" href="https://www.w3.org/TR/css-grid-2/#z-order">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/11827">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  `will-change: z-index` establishes a stacking context on a grid item.
+">
+<style>
+.test {
+  will-change: z-index;
+  width: 100px;
+  background: red;
+}
+.test::before {
+  content: "";
+  display: block;
+  position: relative;
+  z-index: -1;
+  height: 100px;
+  background: green;
+}
+</style>
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div style="display: grid">
+  <div class="test"></div>
+</div>
diff --git a/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-4.html b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-4.html
new file mode 100644
index 00000000000..d8a87b23415
--- /dev/null
+++ b/tests/wpt/tests/css/css-will-change/will-change-stacking-context-z-index-4.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<title>CSS Test: `will-change: z-index`</title>
+<link rel="author" title="Oriol Brufau" href="obrufau@igalia.com">
+<link rel="help" href="https://www.w3.org/TR/css-will-change/#valdef-will-change-custom-ident">
+<link rel="help" href="https://www.w3.org/TR/CSS2/visuren.html#z-index">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/11827">
+<link rel="match" href="../reference/ref-filled-green-100px-square.xht">
+<meta name="assert" content="
+  `will-change: z-index` doesn't establish a stacking context on a non-positioned block box,
+   because `z-index` doesn't apply in that case.
+">
+<style>
+.test {
+  will-change: z-index;
+  width: 100px;
+  background: green;
+}
+.test::before {
+  content: "";
+  display: block;
+  position: relative;
+  z-index: -1;
+  height: 100px;
+  background: red;
+}
+</style>
+<p>Test passes if there is a filled green square and <strong>no red</strong>.</p>
+<div class="test"></div>