Auto merge of #12813 - emilio:hit-test, r=notriddle

dom: getElementsFromPoint does the hit testing on viewport coordinates.

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- [x] `./mach test-tidy` does not report any errors

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We got this wrong (I think it wasn't my fault actually), I was just writing a test for #12777 when I found this.

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -6660,6 +6660,12 @@
             "url": "/_mozilla/mozilla/global.html"
           }
         ],
+        "mozilla/hit_test_pos_fixed.html": [
+          {
+            "path": "mozilla/hit_test_pos_fixed.html",
+            "url": "/_mozilla/mozilla/hit_test_pos_fixed.html"
+          }
+        ],
         "mozilla/htmlcollection.html": [
           {
             "path": "mozilla/htmlcollection.html",