bug#60210: 30.0.50; tab-bar height not recalculated when face changes

[Gregory Heytings]

> > The bug is that, in redisplay_tab_bar, WINDOW_PIXEL_HEIGHT (w) uses the 
> > height of the default face, which is 39 pixels, whereas new_height, 
> > which is computed with tab_bar_height, uses the font of the tab-bar 
> > face (variable-pitch in emacs -Q).  On my system, new_height is (with a 
> > single *scratch* tab) 36 pixels.  Therefore new_height < 
> > WINDOW_PIXEL_HEIGHT (w), when in fact according to the logic of the 
> > code we should have new_height == WINDOW_PIXEL_HEIGHT (w).
>
> I'm not sure I understand how the above causes the tab bar not to be 
> displayed, or become empty.  AFAIU, it just means the frame's 
> change_tab_bar_height_hook will be called.  What did I miss?

I do not fully understand it either yet.  A simpler recipe, which does not 
involve changing fonts:

(set-face-attribute 'tab-bar nil :height 0.5)
(tab-bar-mode 1)

This should display a tiny tab-bar, it displays a white bar instead.

In redisplay_tab_bar, new_height is set to half the height of a canonical 
line (17 and 34 on my system).  Therefore redisplay_tab_bar returns true. 
The next redisplay cycle finds that WINDOW_TOTAL_LINES (w) == 0, and does 
nothing.