display: remember the last pointer position when border trigger is di…

…sabled

We still need to remember the last pointer position when the border
trigger is disabled (a mouse button is pressed) so we don't trigger a
border on the next move when the mouse button is released and we've
moved to a different screen.

src/display.cpp

@@ -216,6 +216,7 @@ void display::handle_events(int) {
 
             if (mask & (Button1Mask | Button2Mask | Button3Mask | Button4Mask | Button5Mask)) {
                 log.debug("mouse button pressed, skipping border detection");
+                last_pos = { root_x, root_y, root };
                 continue;
             }