Scale: make change in window selection work across monitors

When a window is made selected, e.g., when zoomed in using the scroll wheel, make sure that the active monitor is updated, too.

This addresses a bug which may cause the wrong window (a window on another monitor) to be keyboard-closed after scroll-wheel-zooming. See issue #2401.

Caveat: I currently don't have a dual-monitor configuration, so I haven't been able to test how it works with more than one monitor.