Eli Zaretskii <eliz@gnu.org> writes:

> They are numbers, so they can change the width because different
> digits have different width.

No, they don't (in the vast majority of proportional fonts).

>> I don't understand what you mean by that.  Could you explain and give an
>> example?
>
> I've shown different fields for the mode line below:
>
>  -\**-  *mail*         Bot L30    (Mail Fly/en Abbrev Fill) 9:41PM 0.08 [100%]
>  +++++  +----------+   +-+ +-+    +-----------------------+ +----+ +--+ +----+
>
> A single + means a field of 1 character cell; a +---+ means a field
> that starts and ends at the +.  The idea is that each field always
> starts at a predefined pixel offset from the beginning of the mode
> line.  The each field can change its width, but it will not affect the
> following fields.

Here's two random mode lines: