The actual combo box in the ComboBox Chooser should match the width of the widget

[Gold856]

Is your feature request related to a problem? Please describe.
The combo box is always a static size, even when the entire widget is made wider by dragging.

Describe the solution you'd like
The combo box's width should match the width of the widget, possibly with a minimum to prevent a combo box that's too thin.

Describe alternatives you've considered
Not changing anything, but it feels weird to have the combo box always be the same size.

Additional context
It would help workaround #127.