This was kind of mind-boggling, narrowing it down to dark mode,
and then to dark mode *but not CSS*.
I kept narrowing it down, and looked into how dark mode was implemented,
and finally figured this out.
`app.call_later(app.refresh_css)` in `App.watch_dark` causes a timing issue.
I'm basically doing TDD to snapshot testing!
I'm creating tests that don't pass yet, setting up an expectation
that the app match the given screenshots, which is funny in a nice
"improper hierarchy" sort of way, but it's possible because I do
actually have the app rendering how I want, just only in isolation.
If I run the ascii_only tests by themselves, I can get good results
from them, but running them interwoven with default Unicode-using UI
tests doesn't work yet, since the ASCII-only mode permanently changes
how certain widgets render, for the life of the process, so that's
what I'm applying TDD to: making it toggleable at runtime.
I commented out the Unicode tests, and uncommented the ASCII-only tests,
renamed test_snapshots.ambr to test_snapshots_ascii.ambr,
reverted the changes to test_snapshots.ambr to get the Unicode version,
ran my new merge_ambr.py script to join the sets of snapshots,
then replaced test_snapshots.ambr with test_snapshots_merged.ambr
Finally, I uncommented both sets of tests, and I'm ready to do TDD!
First I tried setting PYTEST_TEXTUAL_PAINT_ARGS as an environment variable, to be interpreted by args.py, but it turns out args.py is only executed once, not once per test. It's not using subprocesses, only importing and reimporting the app code, and instantiating new App instances, so parts of the code that are at the top level of modules is only evaluated once.
So I found a new strategy, of importing the `args` object in the test fixture and modifying it directly.
I also realized the --ascii-only option permanently modifies Textual's widgets and borders, and my own widgets, for the life of the process, so I'm holding off on that one. I should be able to make --ascii-only mode more dynamic, and could even target it as a runtime toggle, as a goal, since that's basically what I'll need to achieve to get it working for the tests, but thinking of it as a feature is more fun.
