On Gentoo, updating from xfce-base/xfwm4-4.13.0-r1 to xfce-base/xfwm4-4.13.1 or xfce-base/xfwm4-4.13.1-r1 has changed the default placement of a new window:
Starting the terminator application via shortcut, fullscreen and predefined layout, it was opening at the current screen before. Now it always opens on the left screen of a 3 screen layout:
terminator -l "2x2" -m Shortcut: Shift+Ctrl+Alt+"
However I can't reproduce this with galculator, firefox or even terminator without the -l (layout) -m (maximized).
My settings are:
Window Manager->[x]Focus follows mouse, [x]Automatically give focus to newly created windows
Window Manager Tweaks->Placement->[x]At the center of the screen
Window Manager Tweaks->Focus->[x]Switch to window's workspace, [x] Honor standard ICCCM focus hint
I found that layouts have 'position =' definitions. However the handling must have changed as it seems to be ignored with 4.13.0 as layouts were opening on the current screen.