bug#61948: 29.0.60; window-at returns nil when tab-bar-mode is on

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Fri, 3 Mar 2023 19:02:06 -0800
> 
> To reproduce:
> 
> 1. Type C-x t 2 to turn on tab-bar-mode
> 2. Evaluate (window-at 0 0), it returns nil
> 3. Evaluating (window-at 50 50) also returns nil
> 4. Turn off tab-bar-mode, and (window-at 0 0) returns a live window
> 
> window-at-x-y is the same

What did you expect the function to return instead in these cases?

Both of these functions ignore the tool-bar and the tab-bar
pseudo-windows, so as long as the coordinates are inside those two
windows, both functions return nil.  (This was not called out in their
doc strings; I've now added this caveat to the doc strings of these
two functions.)

So the call with both X and Y zero returns nil because that point is
within the area of the frame where these functions always return nil.

As for the call with X and Y being 50, it returns nil for different
reasons:

  . for window-at, X and Y are measured in columns and rows, so likely
    Y = 50 is outside of the frame created by "emacs -Q";
  . for window-at-x-y, Y = 50 is still inside the tab-bar
    pseudo-window; on my system using Y = 53 or larger returns the
    window showing *scratch*; it could be that the threshold value on
    your system is a bit different

Given the above, do you still think there's a bug here?  If so, please
describe the real use case where you needed this.

Thanks.