MM-64544 Fix width of threads textbox at all screen sizes (#31366)

webapp/channels/src/components/threading/global_threads/global_threads.scss

@@ -65,7 +65,7 @@
 
     /* === Responsiveness === */
     --list: minmax(min-content, 300px);
-    --pane: minmax(min-content, auto);
+    --pane: auto;
 
     display: grid;
     overflow: hidden;