Make all OverlaySheets window-modal
Right now half of Discover's OverlaySheets are view-modal and the other half are window-modal. This feels arbitrary to the user, and view-modal sheets look weird since the global drawer doesn't darken. This is because it's still interactive, but there's really no point in being able to interact with the global drawer while a sheet is open anyway. So let's just make them all window-modal.
BUG: 426065 FIXED-IN: 5.20
(no screenshots because the change looks exactly how you would expect, with the sheet appearing over a darkened area that covers the whole window, not just part of it)