Fix code comment position with Firefox

After selecting a text, we display <gr-selection-action-box> which is
positioned absolutely. Under Firefox, the elements lays somewhere
below its parent element as if it was inline.

Setting `display:block` on the parent causes Firefox to position the
child properly.

https://phabricator.wikimedia.org/T367029

Release-Notes: Fix code commenting popup position under Firefox
Change-Id: Ida52a24553fad486b235226ca6b905ea34646276
(cherry picked from commit 64b80e501a1698ee982d984a79cd74f6cc6f5453)
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-styles.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-styles.ts
index 8b333e6..1de8298 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-styles.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-styles.ts
@@ -75,6 +75,8 @@
   gr-diff-element {
     /* for gr-selection-action-box positioning */
     position: relative;
+    /* Firefox requires a block to position child elements absolutely */
+    display: block;
   }
   gr-diff-element gr-selection-action-box {
     /* Needs z-index to appear above wrapped content, since it's inserted