bug#18429: bug#21469: bug#21348: 25.0.50; Screen scaling factor >=2 causes menus, tooltips to display in the wrong place

[Ryan Prior <ryanprior@gmail.com>]

On Tue, Oct 13, 2015 at 10:51 AM, martin rudalics <rudalics@gmx.at> wrote:

> Are the frame parameters ‘top’ and ‘left’ affected?  Suppose you do say
> (set-frame-parameter nil 'left 500) with scaling in effect.  Does the
> frame appear 500 pixels left of the left screen edge?  If not, then
> mouse warping (‘set-mouse-absolute-pixel-position’) is probably affected
> too and we really have to look into a more generic solution.

I spent some time playing with frame positions.

TABLE: `(set-frame-parameter nil 'left ,x)
_____________________________________________
x       | actual frame distance from left screen edge (px)
0       | 20
500   | 520
1600 | 1620
1800 | 1772
2000 | 1772

A few observations:
1) offset of 20 pixels
I've never noticed this issue because it doesn't affect maximized
frames. Maybe that number 20 is significant somehow, or perhaps this
is a separate bug. The first time after I start `emacs -Q` and set the
left frame edge to 0, the frame flashes momentarily into place flush
with the left screen edge, for perhaps a single video frame, and then
jumps 20 pixels to the right. Subsequent calls to set the left frame
edge to 0 do not trigger this flashing behavior.
2) numbers are proportional, modulo the unexplained offset
We do not see doubling behavior here. I have added no scaling code
pertaining to frame positioning.
3) frame "sticks" to the right screen edge
Given the width of the frame I was testing with, when the left frame
edge is 1772 pixels from the left screen edge, the right frame edge is
flush with the right screen edge. Setting the left frame edge to a
greater value does not result in a further movement of the frame.

l appreciate any help with corroboration and analysis of these results.

Yours,
Ryan