`xfce4-terminal --geometry +0+0` position the top left corner of the terminal on the top left corner of the screen, this is expected.
However `xfce4-terminal --geometry -0-0` position the top left corner of the terminal on the bottom right corner of the screen, whereas it is expected that the bottom right corner of the termial should be placed on the bottom right corner of the screen.
(Similarly `xfce4-terminal --geometry -0+0` position the top left corner on the top right of the screen rather than the top right corner on the top right of the screen.

This is a new bug (from the last fewish months), it was working fine for a long time. I am using an up to date archlinux:
- xfce4-terminal 0.8.6 (Xfce 4.12)
- libvte-2.91.so.0 from the package vte3 0.48.3-1

Maybe related to this bug: https://bugzilla.xfce.org/show_bug.cgi?id=12791
It is probably related to this commit: 93e0123f8b586e07a3fb205cb9c2d1035442d85a

But I admit that I don't understand how this is possible, this commit introduces the handling of gravity by gdk in xfce4-terminal, so it should have fixed the bug I am seeing, whereas in practice
it was working fine before, and now it is now not working correctly anymore.

According to the documentation of gtk-window-move: https://developer.gnome.org/gtk3/stable/GtkWindow.html#gtk-window-move:

"To position a window at the bottom right corner of the screen, you would set GDK_GRAVITY_SOUTH_EAST, which means that the reference point is at x + the window width and y + the window height, and the bottom-right corner of the window border will be placed at that reference point. So, to place a window in the bottom right corner you would first set gravity to south east, then write: gtk_window_move (window, gdk_screen_width() - window_width, gdk_screen_height() - window_height)"

the current code

            if (mask & XNegative)
              {
                x = screen_width - window_width + x;
                gravity = GDK_GRAVITY_NORTH_EAST;
              }
            if (mask & YNegative)
              {
                y = screen_height - window_height + y;
                gravity = (mask & XNegative) ? GDK_GRAVITY_SOUTH_EAST : GDK_GRAVITY_SOUTH_WEST;
              }
            gtk_window_set_gravity (GTK_WINDOW (window), gravity);
            gtk_window_move (GTK_WINDOW (window), x, y);
should be correct.

So it may be a bug in libvte/gdk. I tried to test with gnome-terminal, but gnome-terminal is buggy too: the west/east negative offset works as expected, but the north/south one has a different bug: a -30 offset is interpreted as a +-30, ie place the top of the window -30 pixels above the top of the screen. So gnome-terminal --geometry -10-10 will be interpreted as a --geometry -10+-10 and place the top right corner of the terminal 10 pixels on the left and above the top right corner.