fixed-fill in an editor pane

Using Capi, I made an editor-pane: (make-instance 'capi:editor-pane
:fixed-fill 50).
When this pane is displayed, the fixed-fill feature doesn't occur. I
discovered by accident that if I manually drag the width of the displayed
pane, the fill feature works correctly.
If I then enter more text, the fill doesn't work until I again drag the
width.

A- Is there a way to work around this problem?
B- I don't know any way to programmatically change the size of an already
displayed interface. Is there one?

Thanks,
Jim Bushnell