Fix a weird layering issue with check action tooltips

Change-Id: I1109347f6a506b52e3b92dbe6760381d8a1490ac

polygerrit-ui/app/elements/checks/gr-checks-action.ts

diff --git a/polygerrit-ui/app/elements/checks/gr-checks-action.ts b/polygerrit-ui/app/elements/checks/gr-checks-action.ts
index 82680aa..9fe86c8 100644
--- a/polygerrit-ui/app/elements/checks/gr-checks-action.ts
+++ b/polygerrit-ui/app/elements/checks/gr-checks-action.ts
@@ -41,6 +41,13 @@
           display: inline-block;
         }
         gr-button {
+          /* It is not fully understood why this is needed, but otherwise the
+             paper-tooltip may render under some iron-icons of the content
+             below. Maybe this has to do with a z-index:0 setting for
+             paper-button, such that a stacking context is created. And the high
+             z-index of the paper-tooltip will then only be interpreted within
+             that stacking context. */
+          z-index: 1;
           --padding: var(--spacing-s) var(--spacing-m);
         }
         gr-button paper-tooltip {