fix(gerrit): fix hovercards under Firefox

Firefox doesn't implement the IE6 fromElement/toElement, and it's
not in the MouseEvent spec (at the moment).

Replace with the worse-named but better-specified target and
relatedTarget attributes instead.

Upstream change: https://gerrit-review.googlesource.com/q/I9eeb26c032a38de9d7185749373c7982c796acb2

Change-Id: I9f9a1eb9342bc80b91b5b364a04cc5fa9a7ccaeb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1442
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/gerrit/0007-Fix-gr-hovercard-behavior-under-Firefox.patch

@@ -0,0 +1,35 @@
+From 460d4806feef7fdddc8e3b63dec4fa1854fd2561 Mon Sep 17 00:00:00 2001
+From: Luke Granger-Brown <lukegb@google.com>
+Date: Sat, 25 Jul 2020 23:54:18 +0000
+Subject: [PATCH 7/7] Fix gr-hovercard-behavior under Firefox
+
+Firefox doesn't implement toElement/fromElement (they're not in the
+spec, and were implemented by Chrome apparently for compatibility with
+IE), but does implement target and relatedTarget, which are part of
+the UI Events spec
+(https://w3c.github.io/uievents/#interface-mouseevent).
+
+Bug: Issue 13175
+Change-Id: I9eeb26c032a38de9d7185749373c7982c796acb2
+---
+ .../app/elements/shared/gr-hovercard/gr-hovercard-behavior.js | 4 ++--
+ 1 file changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
+index 04c3166dd5..3d5937b1fa 100644
+--- a/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
++++ b/polygerrit-ui/app/elements/shared/gr-hovercard/gr-hovercard-behavior.js
+@@ -209,8 +209,8 @@ export const hovercardBehaviorMixin = superClass => class extends superClass {
+     // from the hovercard but now hovering over the target (to stop an annoying
+     // flicker effect), just return.
+     if (opt_e) {
+-      if (opt_e.toElement === this ||
+-          (opt_e.fromElement === this && opt_e.toElement === this._target)) {
++      if (opt_e.relatedTarget === this ||
++          (opt_e.target === this && opt_e.relatedTarget === this._target)) {
+         return;
+       }
+     }
+-- 
+2.27.0
+