I found https://github.com/wikimedia/oojs-ui/commit/65fb50d22dc1187f4fe70c3c530dfa7ed1d3e328 as the commit that introduced fixed width. (Ref T411252)

@Esanders, What is the recommended way to override this fixed width? Or should CX usecase should be considered in the upstream fix?

You have a downstream override to make that tool full width:

.ve-cx-toolbar-mt .ve-ui-toolbar-group-cx-mt {
  display: block;

You'll need to update your downstream fix to remove the width limitation on the VE toolbar. This rule needs overriding (for your specific tool)

.oo-ui-menuToolGroup .oo-ui-popupToolGroup-handle {
  width: 140px

There are also extra left and right borders on the menuToolGroup that should probably be removed.

Change #1294492 had a related patch set uploaded (by Santhosh; author: Santhosh):

[mediawiki/extensions/ContentTranslation@master] fix: MT provider dropdown label appear as clipped

https://gerrit.wikimedia.org/r/1294492

Change #1294492 merged by jenkins-bot:

[mediawiki/extensions/ContentTranslation@master] fix: MT provider dropdown label appear as clipped

https://gerrit.wikimedia.org/r/1294492

LGTM