Function: get-buffer-window

Return a window currently displaying BUFFER-OR-NAME, or nil if none.
BUFFER-OR-NAME may be a buffer or a buffer name and defaults to
the current buffer.

The optional argument ALL-FRAMES specifies the frames to consider:

- t means consider all windows on all existing frames.

- `visible' means consider all windows on all visible frames.

- 0 (the number zero) means consider all windows on all visible
and iconified frames.

- A frame means consider all windows on that frame only.

Any other value of ALL-FRAMES means consider all windows on the
selected frame and no others. (fn &optional BUFFER-OR-NAME ALL-FRAMES)