Hello,

I found that with 'mode-line-compact' being non-nil, mouse highlighting
would not work properly. To reproduce:

  - emacs -Q
  - (setq mode-line-compact t) or (setq mode-line-compact 'long)
  - Hover mouse cursor over the major mode part on the mode line

Now the minor mode lighters are also highlighted. That is presumably
because when 'mode-line-compact' is non-nil, the whole mode line string
is displayed at once. So the glyphs produced all have the same 'object'
field. That confuses the computation of boundaries for mouse
highlighting.

In this patch we instead split the mode line string into elements and
display them one by one.

Thanks!