ui/gtk: Fix relative mouse with multiple monitors

To handle relative mouse input the event handler needs to move the mouse
away from the screen edges. Failing to do so results in the mouse
getting stuck at invisible walls. However the current implementation for
this is broken on hosts with multiple monitors.

With multiple monitors the mouse can be located outside of the current
monitor which is not handled by the current code. Also the monitor
itself might be located at coordinates different from (0, 0).

Signed-off-by: Dennis Wölfing <denniswoelfing@gmx.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210720143940.291413-1-denniswoelfing@gmx.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/gtk.c

@@ -865,37 +865,25 @@ static gboolean gd_motion_event(GtkWidget *widget, GdkEventMotion *motion,
         GdkWindow *win = gtk_widget_get_window(widget);
         GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
         GdkRectangle geometry;
-        int screen_width, screen_height;
 
         int x = (int)motion->x_root;
         int y = (int)motion->y_root;
 
         gdk_monitor_get_geometry(monitor, &geometry);
-        screen_width = geometry.width;
-        screen_height = geometry.height;
 
         /* In relative mode check to see if client pointer hit
-         * one of the screen edges, and if so move it back by
-         * 200 pixels. This is important because the pointer
+         * one of the monitor edges, and if so move it back to the
+         * center of the monitor. This is important because the pointer
          * in the server doesn't correspond 1-for-1, and so
          * may still be only half way across the screen. Without
          * this warp, the server pointer would thus appear to hit
          * an invisible wall */
-        if (x == 0) {
-            x += 200;
-        }
-        if (y == 0) {
-            y += 200;
-        }
-        if (x == (screen_width - 1)) {
-            x -= 200;
-        }
-        if (y == (screen_height - 1)) {
-            y -= 200;
-        }
-
-        if (x != (int)motion->x_root || y != (int)motion->y_root) {
+        if (x <= geometry.x || x - geometry.x >= geometry.width - 1 ||
+            y <= geometry.y || y - geometry.y >= geometry.height - 1) {
             GdkDevice *dev = gdk_event_get_device((GdkEvent *)motion);
+            x = geometry.x + geometry.width / 2;
+            y = geometry.y + geometry.height / 2;
+
             gdk_device_warp(dev, screen, x, y);
             s->last_set = FALSE;
             return FALSE;