Hi emacs-devel,

TL;DR: How do I create a frame without a border, in the style of x_create_tip_frame (called by x-show-tip in the C sources), from Elisp?

I'm looking at ways in which we could use real tooptip popups instead of overlays to display company-mode completion lists. This would have many advantages, including better interaction with font sizes, no issues with recursive display specs, good support for variable-width fonts, and so on. I believe Stefan has spoken in favour of that approach in the past, too.

The natural candidate for this task is x-show-tip, and it probably meets most of the requirements; but not all. In particular, there can be at most one tooltip displayed at any time; this means that we'd break other packages that display tooltips while completion is ongoing. Examples include company-quickhelp, which displays documentation for the currently selected entry next to completion candidates.

`x-show-tip` is currently implemented in C; as far as I can see, however, it essentially creates a frame and displays it at a given location; given this, I thought it would be possible to reimplement it in ELisp. Unfortunately, I've been hitting a wall when it comes to displaying a borderless frame. I asked on https://emacs.stackexchange.com/questions/20167/how-do-i-create-a-borderless-frame, to no avail. 

My original guess was that the relevant part of the C code was this call:

  x_default_parameter (f, parms, Qborder_width, make_number (0), "borderWidth", "BorderWidth", RES_TYPE_NUMBER);

Unfortunately, neither (set-frame-param (selected-frame) 'border-width 0) nor (make-frame '((border-width . 0))) (both inspired from the call above) yield a borderless frame.

Is there a way to create a borderless frame from Elisp?

Clément.