@RobertRoeb Thanks! I haven't fully understood the sequence yet, where would it be recreated with pure text, I thought that SetBitmap() would call the implementation's SetBitmap without first re-creating something. But since it happens to you, then I understand that unconditionally changing the style make sense. But I'd also prefer to call the common SetBezelStyleFromBorderFlags() to keep all our HIG wisdom at one place and consistent, would just calling this instead of manipulating the style directly also solve your problem ?

According to the docs NSRegularSquareBezelStyle and NSSmallSquareBezelStyle are perfect for bitmaps and other cases because they can scale to any size, while NSRoundedBezelStyle a.k.a NSBezelStylePush are for text content, so I think it's decision tree is still valid. Otherwise I'd prefer to fix it there.

Yes, that works as well. While testing that, I noticed that when changing the button from "play" to "pause", since "play" is shorter, "pause" gets wrapped. So can we invalidate the size calculation somehow?
[Edit myself]: I realize that I was under the assumption that the control would resize itself after setting a new, longer label and DoGetBestSize() correctly reported the new size. But I actually had to call the parent window's wxSizer::Layout() for the button to change size. This, sort of, makes sense, but I anticipated an immediate change of the button size. Anyways.