view: the code pane can replace text the user typed #2066

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Problem

Text typed in the code pane can be replaced by the host's copy of the file.

adoptModels calls syncEditor in web/view/src/main.ts, which calls editor.show(file, hostText) in web/view/src/editor.ts. show cancels the pending debounce and replaces the document with the host's text. This loses typing in these cases:

• In compare mode, text typed before the base ref has arrived from GitHub.
• Text the host rejected because it did not compile yet, when a later model update (a cell edit, for example) replaces it.
• A pending change when the user switches files within the 350 ms debounce.

Proposed fix

• Record the text last sent to and received from the host, and do not replace the document while it differs from that text.
• Flush the pending change before switching files.