Layered display API (was: bug#18195: 24.3.92; window-screen-lines is not accurate)

[Dmitry Gutov <dgutov@yandex.ru>]

On 08/06/2014 06:52 AM, Eli Zaretskii wrote:

 >> That sounds great, but I don't even know where to start. What we're
 >> trying to emulate, though, is multi-layered display. That sounds
 >> somewhat different from a "text layout API" to me.
 >
 > I don't mind how it is called and what is the correct abstraction.  I
 > do know that we need some abstraction and APIs to support it.  If you
 > already know what the correct abstraction is, then you are in a good
 > position to formulate the requirements.

I don't know much of the other use cases (only in-buffer calltips come 
to mind).

Basically, we'd like to be able to display a rectangle with propertized 
text inside, at an arbitrary position (I would say pixel coordinates, 
but that might not work well in terminal), so that it would be displayed 
above the buffer contents.

This would solve two issues we currently have:

1. If the buffer ends (shortly) after the current line, we're forced to 
pad it with a newline, and then carefully undo that and restore the 
buffer modification status.

2. If the buffer is already heavily using the `display' text property, 
or other similar ones, our tooltip positioning also breaks or works 
unexpectedly. Example: the `report-emacs-bug' buffer 
(https://github.com/company-mode/company-mode/issues/136).

Somewhat relatedely, I'd love to be able to sanely display smooth 
graphics spanning multiple lines in the fringe (or in the area that 
would replace it), to indicate the buffer lines that changed from the 
last VCS commit, without conflict with flymake-style warning/error 
indicators. Code folding packages might also use it to indicate the 
bounds of the function or block at point. Maybe even support buttons?

Here's an illustration from a proprietary editor: 
http://wiki.jetbrains.net/i/images/6/68/Git_general_branches_updated_code.png