Bug 1856252 - Set a smaller minimum size for tree view columns. r=arschmitz

Info and explanation of the change are in the bug.

Differential Revision: https://phabricator.services.mozilla.com/D215813

--HG--
extra : amend_source : 7c56a6ac09f4d6cb9266c4611be3986a60a3b26f

mail/themes/shared/mail/tree-listbox.css

@@ -420,7 +420,7 @@ thead[is="tree-view-table-header"] {
 
 th[is="tree-view-table-header-cell"] {
   height: var(--tree-header-table-height);
-  min-width: 7ch;
+  min-width: 5ch;
   padding: 0;
   position: relative;
   overflow: hidden;