If you drag-select text with your cursor from left to right in the sidebar, say from the description, it will prompt snap.js of the left navigation to move the content to the right, leading the right sidebar to disappear.
That's one reason why we don't use snap.js on desktop, only on mobile. ;)
We should probably fix this in core, but as Deck right now is the only app which overrides the mobile handling I'm opening it here. Probably a simple removal of the mechanic of content getting dragged to the right too far would fix it?