Bug 1807233 - Make treecol labels crop again. r=Gijs

This is the lower-risk fix. Ideally we'd move all the places windows etc
to use modern flex layout, but that might be worth doing at some other
time.

Differential Revision: https://phabricator.services.mozilla.com/D165489

toolkit/themes/shared/tree.css

@@ -135,6 +135,9 @@ treecol:where(:hover),
 
 .treecol-text {
   margin: 0 !important;
+  /* NOTE(emilio): Eventually we should move <treecol> out of XUL layout
+   * proper, till then this makes cropping work. */
+  -moz-box-layout: flex;
 }
 
 treecol[hideheader="true"] {