[Glitch] Fix invisible username during post highlight animation

Port 1f1e91a21526ea84e5b7e5e3d6fdaff047fcda5c to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/styles/mastodon/components.scss

@@ -1680,6 +1680,7 @@ body > [data-popper-placement] {
     content: '';
     position: absolute;
     inset: 0;
+    z-index: -1;
     background: var(--color-bg-brand-softest);
     opacity: 0;
     animation: fade 0.7s reverse both 0.3s;