bug#12419: Mouse click changes layout

[martin rudalics]

> Where in the code or the infrastructure do we enforce an integral
> number of lines in a window?

All over the window handling code, presently.

> AFAIK, the only restriction imposed by the display engine is that a
> window's first line must be completely visible (unless its height is
> larger than the window).  But the last line of a window can be only
> partially visible.  That seems to imply that you should be able to
> split a window such that each child gets exactly half, in pixels.

I'm afraid that many people wouldn't want that.  So even if we manage to
provide really maximized frames, the window handling code will have to
show most windows with fully visible lines.

> What am I missing?

Not much, I suppose.  After the freeze we can set up a branch for
implementing frame and window sizes in pixels.  The display routines
will then have to be able to handle frame and window sizes specified in
pixels.  And I suppose that we want a function that calculates the
number of pixels between two buffer positions to get rid of the current
post processing mess in `fit-window-to-buffer'.

martin