Naturally, 5 minutes after posting to the forums, I found the solution on accident. Noob. Posting the result in case anyone else has this issue...
The sidebar is resizable by placing the mouse cursor over the divider and click+drag to resize. Normal operation doesn't allow me to resize so small as to completely collapse the side panel (which was the issue), so I don't know what I did to fully collapse the side panel, but it can be restored by simply click+drag back to normal size even if it is collapsed. The mouse cursor should change to a horizontal reposition cursor when hovering over the divider, even if it is at the far left of the screen; this is what clued me in on the solution.