so, I looked in mainsession.C while hunting about a_x11_host callsites
see what I found:
void MainSession::default_window_positions(int window_config)
{
// 0 - all windows on a, playback_config a
// 1 - all windows on b, playback_config b
// 2 - all windows on a, except composer on b, playback_config b
but in reality this config set everything on A (Xephyr screen)
A_X11_HOST :1.0
B_X11_HOST :0.0
WINDOW_CONFIG 2
this config set everything BUT compositor on A
A_X11_HOST :1.0
B_X11_HOST :0.0
WINDOW_CONFIG 1
this config set only compositor on B:
A_X11_HOST :0.0
B_X11_HOST :1.0
WINDOW_CONFIG 1
and this config se ALL on A
A_X11_HOST :0.0
B_X11_HOST :1.0
WINDOW_CONFIG 0
so check your Cinelerra_rc for WINDOW_CONFIG value!
