Bug 1642364 - Add a temporary solution for modal dialog centered alignment r=emilio

Spec: https://html.spec.whatwg.org/multipage/#centered-alignment

The spec is opaque about the centered alignment mode and we couldn't
come up a satisfied solution by using the current version of the
spec. There will be discussion in CSSWG to talk about better solutions.
In the meantime, we are adding this patch as a temporary solution with
basic centered alignment support to allow us enabling the feature
in Nightly for more testing.

Differential Revision: https://phabricator.services.mozilla.com/D77678

layout/style/res/html.css

@@ -828,6 +828,7 @@ dialog {
   padding: 1em;
   background: white;
   width: -moz-fit-content;
+  height: -moz-fit-content;
 }
 
 dialog:not([open]) {
@@ -836,6 +837,13 @@ dialog:not([open]) {
 
 dialog:-moz-modal-dialog {
   -moz-top-layer: top !important;
+  /* This is a temporary solution until the relevant CSSWG issues are resolved */
+  position: fixed;
+  max-height: 100vh;
+  max-width: 100vw;
+  top: 50%;
+  transform: translateY(-50%);
+  overflow: auto;
 }
 
 /* https://html.spec.whatwg.org/#flow-content-3 */

testing/web-platform/meta/html/semantics/interactive-elements/the-dialog-element/abspos-dialog-layout.html.ini

@@ -1,34 +1,11 @@
 [abspos-dialog-layout.html]
-  [showModal() should center in the viewport]
-    expected: FAIL
-
-  [The dialog is a positioned element, so the top and bottom should not have style auto.]
-    expected: FAIL
-
-  [Dialog should be recentered if showModal() is called after close()]
-    expected: FAIL
-
-  [Dialog should not recenter on relayout.]
-    expected: FAIL
-
-  [A tall dialog should be positioned at the top of the viewport.]
-    expected: FAIL
-
+  prefs:[dom.dialog_element.enabled:true]
   [The dialog should be centered regardless of the presence of a horizontal scrollbar.]
     expected: FAIL
 
   [Centering should work when dialog is inside positioned containers.]
     expected: FAIL
 
-  [A centered dialog's position should survive becoming display:none temporarily.]
-    expected: FAIL
-
-  [Dialog should lose centering when removed from the document.]
-    expected: FAIL
-
-  [Dialog's specified position should survive after close() and showModal().]
-    expected: FAIL
-
   [Dialog should be recentered if showModal() is called after removing 'open'.]
     expected: FAIL