White Triangle / Arrow on All Edits in Timeline

I pressed some combination of keys accidentally and now all edits (Ctrl-K or with a Razor tool) get weird triangles as show below. I google around and don't see anyone mentioning them. They seem to have no effect. They're not draggable in any way. I looked at the View menu to see if something was turned on there. Nope. It's a funny situation, but I'm hoping someone knows what these are and how to turn them off.