Don't try to resize popup attached to compose window button

I don't know if this ever worked, but at least in the current version
of Thunderbird, when we try to resize the popup attached to the
compose window what we end up resizing is the compose window itself.
Don't do that.

ui/popup.js

@@ -1178,7 +1178,7 @@ const SLPopup = {
 
     SLPopup.restoreZoom();
     SLPopup.initialized = true;
-    SLTools.makeContentsVisible();
+    if (!SLPopup.attachedWindow) SLTools.makeContentsVisible();
     SLTools.debug("Initialized popup window");
   },
 };