Problem: with a float focused and the target buf only shown in the
last non-float window, do_buffer_ext goes down the buf != curbuf
path. close_windows can't touch the last non-float, b_nwindows stays
> 0, close_buffer is skipped, returns OK silently.
Solution: if a non-float still holds buf after close_windows, jump
into it and recurse. Then buf == curbuf and the existing replacement
path takes over.
54f22a8f01