Problem:
(Followup to 54f22a8f01c0feb27a531b52aedf5cdbd5e51b24.)
Deleting another buffer from a floatwin could move focus into the holder
window and fire BufEnter for the buffer being deleted.
Solution:
Use switch_win_noblock() instead of buf_jump_open_win() before
recursing into do_buffer_ext().
26df751add