Wow, three years later and believe it or not, this tool lives on! It's actually an incredibly complex and comprehensive tool used extensively on a massive engineering program. My tiny test program here is but a fraction of a fraction of a simplified snippet of the code!

Anyway, I'm updating the program now and think I've figured out a solution. I wanted to post here in case it helps others.

The $adjuster->slave->GeometryRequest indeed seems to be an answer, albeit there could still be a more elegant solution out there.

By giving GeometryRequest the arguments of ($adjuster->parent->width * 0.2, $adjuster->slave->height) I seem to have gotten it to behave as desired, at least in the full program!

I'm not sure why this works exactly, but assigning the adjuster 20% of the available width seems to help? I should mention there is another method that gets called immediately after, which resizes the column widths in what is actually an MListbox, to occupy the entire available space. However, I was seeing the same problematic behavior without the GeometryRequest, and it seems to work with it!