FvwmIconMan won't correctly apply colorsets when swallowed

[somiaj]

When FvwmIconMan is swallowed in FvwmButtons and a colorset is used as the background color and FvwmIconMan is set to grow with number of buttons, the background won't be filled in the unused area. See the black area in the screenshot here.

http://sobalvarro.co/fvwm.png

Manually coding the background color works, but using a colorset does not.

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