The completion component assumes that it operates on the same View but
it's possible to break this assumption by switching windows through
left-clicking. I believe we should clear the completion menu when
switching windows to fix this.
This change fixes a panic for this scenario:
* Open a buffer with LSP completion available
* Split the window (for example '<C-w>v')
* Enter insert mode and trigger the completion menu
* Select a completion candidate (for example with '<C-n>')
* Switch to the original window by left-clicking in its area
* Enter insert mode and make edits (for example 'o<backspace>')
This will trip the 'assert_eq' in Document::restore.
48373d4a2b