From gerrit:

I run into this while working on Ie0334df (T260157). In VisualEditor, InputWidget.setValue() is called on construction time: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/VisualEditor/+/refs/heads/master/modules/ve-mw/ui/pages/ve.ui.MWParameterPage.js#49
These widgets are not disabled (they shouldn't), but not attached to anything yet. Still ComboBoxInputWidget.onInputChange() is triggered (this is intentional, as far as I can tell). This unconditionally calls MenuSelectWidget.toggle( true ). This prints a warning and causes the combobox widget to behave odd. The dropdown menu is open when the form opens, but is not properly sized.