I have this form, where the light blue is a flow layout panel and the white is a panel inside it.
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It looks like this when running:
If you look carefully, you can see a small border around the edge of the white section.
Why doesn't the panel fill the space inside the flow layout panel? I haven't done any coding; I just saw a guy placed the panel over and it automatically filled the space.
It seems that the margin was bugging it. I fixed it by setting the margin to 0, it was 3 by default. Thanks to Jimi.