This is on Manjaro Linux x64 with XFCE 4.13
In a dual monitor set-up Libreoffice correctly opens on the display (screen) where the mouse pointer is, as expected.
However most 'child windows' such as 'Options', 'Edit > Find and Replace', 'Edit Style', 'Format > Paragraph', 'Format > Character', open always on the external display (see below for details).
On the other hand e.g. 'Format > Columns' always opens on the primary display.
Finally some child windows such as 'Tools > Spelling', 'Table > Insert Table', 'Edit > Go to page', seem to 'follow' the main window (i.e. open on the same display where the main window is located).
If this helps the xranr command used to set-up the displays is the following:
xrandr --output HDMI2 --off --output HDMI1 --off --output DP1 --mode 1920x1080 --pos 0x0 --rotate normal --output eDP1 --primary --mode 1366x768 --pos 1920x0 --rotate normal --output VIRTUAL1 --off
The display where Libreoffice is in this case is 'eDP1' (a laptop monitor) and
I cannot test this, but looking at Bug 113049 which has a different issue it seems this may not be about LO.
Or, if this is LO fault, it's related to Bug 101641.
I do guess that this is indeed related to #101641 albeit in this case e.g. highlighter menu is displayed correctly.
As an additional info, in my case running with:
shows windows on the correct monitor (i.e. where libreoffice is), but at the same time in this case libreoffice is always started on the primary monitor by default.
I am assuming the default gui rendering on here (XFCE) is GTK.
Do note, however, that if I run both gtk2-demo and gtk3-demo all the child windows open as expected on the same display as the main window.
Specifically to Linux and Gtk maybe this (and related links) could be useful?
I don't have Linux for multimonitor but I'll confirm. This may be a duplicate of Bug 101641 which is for Windows, or not. Let's keep it separated so far.
Any chance this could be tested with older versions of LibreOffice ?