First off, we'll add a button to the Selection Launcher to toggle the visibility of the dashed-line marquee of the selection. In computer graphics, this is called marching ants. While it makes seeing selections easy, sometimes it's more of a distraction; especially when we're drawing inside a selection. So here we go...
Choose the Rectangular Selection tool.
Select an area that's slightly larger than the panel. We want to do this just to have a bit of extra pixels selected. Better to do it when we don't need to than to not do it when we need to. Even though the masking on panels hides whatever we draw that's beyond the panels, it's still there. So by selecting a bit outside the panels, we're selecting hidden parts of our drawing. Now we have a bit of extra area to play around with.
Looking at the bottom of the selection, there's a horizontal button bar. That's the Selection Launcher.
