Bug 1586444 - Avoid styling scrollbar part children of object elements with vertical-align. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D53124

--HG--
extra : moz-landing-system : lando

layout/style/crashtests/1586444.html

@@ -0,0 +1,15 @@
+<style>
+#a {
+  overflow: auto auto;
+  display: -webkit-box;
+}
+</style>
+<script>
+window.onload = () => {
+  a.align = "ABSMIDDLE"
+  document.documentElement.style.display="none"
+  document.documentElement.getBoundingClientRect()
+  document.documentElement.style.display=""
+}
+</script>
+<object id="a">

layout/style/crashtests/crashtests.list

@@ -312,3 +312,4 @@ load 1581579.html
 skip-if(release_or_beta) pref(dom.paintWorklet.enabled,true) pref(dom.worklet.enabled,true) load 1593766.html # bug 1581896
 pref(layout.css.motion-path.enabled,true) load 1594949.html
 pref(layout.css.motion-path.enabled,true) pref(layout.css.individual-transform.enabled,true) load 1594960.html
+load 1586444.html

toolkit/content/minimal-xul.css

@@ -73,6 +73,11 @@ scrollcorner, resizer, scrollbar, scrollbarbutton, slider {
    */
   justify-items: start;
 
+  /* Avoid `object > *` rule in html.css from setting a useless, non-initial
+   * value of vertical-align.
+   */
+  vertical-align: initial !important;
+
   /* Duplicate the rules from the '*' rule above, which were clobbered by the
    * 'all: initial' declaration.
    *