bug#21173: 25.0.50; New frames positioned off screen with multiple monitors

[Francis Litterio]

On Windows, using Emacs built from the latest source, I see the following
behavior on a machine with two 1920x1080 monitors arranged side-by-side, such
that the right monitor is the primary monitor (i.e., the monitor on which the
taskbar is drawn).  This monitor arrangement gives frames on the left monitor
negative X offsets, which is why this bug happens.

  $ runemacs.exe -Q

In buffer *scratch*:

  (setq f (make-frame '((left . -1))))
  #<frame emacs <at> IZSYSTEM023 015257f0>

  (frame-parameter f 'left)
  3155

The left offset of the new frame appears to be 1920 pixels too far to the right.
Evaluating the following form positions the frame to its expected location
(abutting the right edge of the right monitor):

  (set-frame-position f (- 3155 1920) 0)

The root cause appears to be that function x_calc_absolute_position (in
xterm.c and w32term.c) calls function x_display_pixel_width to obtain the
combined pixel width of all monitors, but the code doesn't take into account
that one (or more) monitors might have negative X offsets, which can happen on
Windows.  Thus, the new frame's left offset is miscomputed.

Under the X window system, I'm not sure if monitors can have negative X offsets,
so this may not be an issue under X.

I suspect the same problem would happen when making a frame with a 'top
parameter of -1 when the non-primary monitor is positioned above the primary
monitor (i.e., has negative Y offsets).
--
Fran Litterio
flitterio <at> gmail.com



In GNU Emacs 25.0.50.4 (i686-pc-mingw32)
 of 2015-06-30 on PUPPY
Repository revision: 5f004117f5bcab9171eaddb2867393ed69ae49bf
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=c:/apps/emacs --without-x --without-xpm
 --without-png --without-jpeg --without-tiff --without-gif'

Configured features:
SOUND NOTIFY ACL TOOLKIT_SCROLL_BARS

Important settings:
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Major mode: Text

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