[Glitch] Fix compose textarea scroll behavior

Port 746979f75d to glitch-soc

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

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

@@ -1234,6 +1234,10 @@ $ui-header-height: 55px;
 }
 
 .compose-form__highlightable {
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  flex: 0 1 auto;
   border-radius: 4px;
   transition: box-shadow 300ms linear;