Ensure we drop the last pane item

Previously, we weren't updating the toolbar's active item when emptying out a pane. This enhances an integration test to ensure that we don't hold references to any editors or buffers once we close everything.
4f4364d510Nathan Sobo committed on 4/5/2022, 7:05:14 PMparent41a27e6
3 files changedLine totals unavailable