>
> Bottom line: string-pixel-width is hardly efficient enough when
> invoked in a tight loop, so if you need to call it many times for many
> strings, perhaps a better idea would be to insert all the strings into
> a work buffer, then measure their width one by one using
> window-text-pixel-size, each time calling it with different FROM and
> TO values?  That involves switching a window's buffer and then
> switching it back, but maybe it will be faster?
>
> Alternatively, if you only need the width of the widest string in a
> collection, you could concatenate them separated by newlines, then
> call string-pixel-width on the result.
>

It took me a while to understand what this means, that's some leaky
abstraction we have here :)

I've found a solution after some refactoring. The simplest solution is just
don't use relative :align-to. As long as it's not used,
`string-pixel-width` will behave as expected.

Thanks for all your help.