A page from before a deploy reloads for a missing chunk instead of going black

[andrin-n-dream]

Opening a Markdown file sometimes blacked out the whole screen. The
Markdown renderer is a lazy chunk, and every deploy rebuilds web/dist
under new chunk names; a page left open across one asks for the old
name the first time it needs that chunk. The server answered the
missing file with index.html and a 200, the browser refused HTML as a
module, the lazy import threw, and with no error boundary anywhere
React unmounted the whole app. Reproduced on a scratch instance by
answering the chunk that way: #root was left with no children.

Three layers, outermost last:

• lazyChunk wraps React.lazy for both lazy panes (the editor and the
  Markdown renderer): a chunk that fails to load reloads the page once
  per 30s window, 600ms later so the click's debounced ui write lands
  first -- without that the reload came back with the panel open and no
  file in it. A second failure inside the window is real and throws.
• An ErrorBoundary around the files panel's content, keyed by mode and
  path, says 'This could not be shown' with a Reload button and costs
  that pane alone; one around the app says so instead of going black.
• The server answers a missing /assets/ file 404 rather than
  index.html, so the failure says the file is gone.

Measured: with the chunk gone once, the page reloaded once and showed
the file, 3 out of 3; gone for good, it reloaded once and then the pane
said why with all 7 windows intact. The tests fail with the reload
removed and with the window removed.

🤖 Generated with Claude Code