Auto merge of #25715 - mrobinson:position-fixed, r=SimonSapin

Improve position:fixed support in layout_2020

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---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -1,5 +1,14 @@
 {
  "items": {
+  "conformancechecker": {
+   "css/position_fixed_scroll.html": []
+  },
+  "crashtest": {
+   "css/position_fixed_scroll.html": []
+  },
+  "manual": {
+   "css/position_fixed_scroll.html": []
+  },
   "reftest": {
    "css/abs-overflow-stackingcontext.html": [
     [
@@ -5057,6 +5066,18 @@
      {}
     ]
    ],
+   "css/position_fixed_scroll.html": [
+    [
+     "css/position_fixed_scroll.html",
+     [
+      [
+       "/_mozilla/css/position_fixed_scroll_ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/position_fixed_simple_a.html": [
     [
      "css/position_fixed_simple_a.html",
@@ -8977,6 +8998,10 @@
    "css/position_fixed_overflow_b.html": [
     []
    ],
+   "css/position_fixed_scroll.html": [],
+   "css/position_fixed_scroll_ref.html": [
+    []
+   ],
    "css/position_fixed_simple_b.html": [
     []
    ],
@@ -11253,6 +11278,7 @@
      {}
     ]
    ],
+   "css/position_fixed_scroll.html": [],
    "css/stylesheet_media_queries.html": [
     [
      "css/stylesheet_media_queries.html",
@@ -12596,6 +12622,12 @@
      {}
     ]
    ]
+  },
+  "visual": {
+   "css/position_fixed_scroll.html": []
+  },
+  "wdspec": {
+   "css/position_fixed_scroll.html": []
   }
  },
  "paths": {
@@ -16959,6 +16991,14 @@
    "a8947566153c187ba3191084f89d0163bc5b666e",
    "support"
   ],
+  "css/position_fixed_scroll.html": [
+   "712265957f2c2ff0102eab09219b44bc5bf7f032",
+   "reftest"
+  ],
+  "css/position_fixed_scroll_ref.html": [
+   "dc2e72a96ac2d82f038605812c79b9f78c1547dd",
+   "support"
+  ],
   "css/position_fixed_simple_a.html": [
    "d01b955091107990d014e7f7be9c1181e5b06d66",
    "reftest"