bug#55207: 29.0.50; tab-bar-format-align-right only works correctly with monospace characters

[Juri Linkov]

> However, in the more complicated configuration I actually use it doesn't
> make much of a difference:
> ...
> The icons on the mode line are small XBM images, the theme is Modus
> Operandi.

I guess you are using global-mode-string with XBM images.
Could you then try something like this to see what number it returns?

(string-pixel-width (string-trim-right (format-mode-line global-mode-string)))