Re: Fill column indicator functionality

[Stefan Monnier]

>> Being its own terminal emulator, Emacs could do the same to avoid gaps in
>> horizontal and vertical lines that should connect.
>
> Unfortunately, this is not currently possible, not without significant
> changes in how the Emacs display engine works internally.  The display
> routines are currently called in a way that doesn't allow them to have
> a window-global view of the display, not even a way of knowing what's
> on the previous and the next screen lines.  In fact, the display
> routines need to do their job even if the information about the layout
> of adjacent screen lines is not available at all, and redisplay
> optimizations frequently make use of this property, by starting a new
> redisplay cycle on a specific line, doing part of the layout, then
> throwing out the results.  About the only thing the display routines
> can rely on is the window-start point and the window pixel
> dimensions.  Even the vertical coordinate of a line on which the
> display routine was called is frequently unknown. 

I think what Mattias is referring to is to change the part of the code
where we draw the glyph matrix onto the screen (i.e. the part that's
window-system specific), to add ad-hoc hacks that try to display
box-drawing characters differently to try and fill small gaps
between them.  I don't know how workable that would be, especially when
considering proportional fonts (i.e. the problems with box-drawing
characters aren't limited to gaps between them but also include
misalignment).


        Stefan