Scrolling compositors: side struts make neighbor columns peek

In umbriel (and niri) scrolling layout, layout.struts.left/right shrink the area columns are laid out in but the strip still renders across the whole output, so the next column shows in the strut space. To align windows with a floating bar, match the bar's side margin to the layout gap instead (window border outer edge = gap logical px from the output edge) and use only struts.top for the bar-to-window spacing.