Hi Jeff,
Two things;
a) switch the browse & form layouts from Flex-Div to Flex-Grid. (in WebServer procedure)
b) Also in web server procedure, set the CSS to "reset to defaults". ie;
In WebServer extension, Settings, Styles, then all tabs there, tick (and leave on) "Reset to Defaults".
this will alight the global css settings to the current recommended global css settings.
then in MailBoxesBrowseControl, Children tab, TestSideMenu child, set "Start Column (grid only)" to 2, and span column to 1.
(I don't think the above does anything in this situation, but it's more correct.)
At this point you'll see the browse/menu fill the popup, so make the popup as wide as you like.
You'll also notice the browse is "centered" vertically, that's because of the css
align-items:center;
which is on the containing form cell div. You can put custom css in there to align-items top if you like.
Cheers
Bruce