Fix pop-in player placeholder layout in thread view (#24715)

app/javascript/styles/mastodon/components.scss

@@ -1155,6 +1155,7 @@ body > [data-popper-placement] {
     .video-player,
     .audio-player,
     .attachment-list,
+    .picture-in-picture-placeholder,
     .status-card {
       margin-inline-start: 46px + 10px;
       width: calc(100% - (46px + 10px));