Fix/autoreload watcher race

[Felipenguim]

📝 Summary

Closes #10191
Related: #10225 (found while validating this fix)

The bug

marimo's module autoreload has two independent consumers sharing the same ModuleReloader state (modules_mtimes / stale_modules in marimo/_runtime/reload/autoreload.py). Both call ModuleReloader.check(), which one sees a file's mtime change first "consumes" it for both, creating a race conditioncell_scope() reloads the module correctly, but the watcher never gets to run its transitive dependency analysis for that change. like the self-import cycle in #10191 — never get marked stale.

The fix

ModuleReloader gets a second, independent mtime baseline (watcher_modules_mtimes) and a check_for_watcher() method, used only by ModuleWatcher. This lets the watcher detect "did this file change since I last checked" on its own terms, regardless of whether cell_scope() already reloaded it in the meantime. _check_modules now calls both check() (kept — cell_uses_stale_modules/flag_if_imports_stale still rely on it being populated) and check_for_watcher() (used for the transitive-dependency computation).

Tests

• test_reload_self_import_cycle: the scenario from the issue.
• test_reload_self_import_cycle_race_with_other_cell: same setup, but forces an unrelated cell to rerun right after the file changes and before the watcher's next poll. Specifically exercises the race.

Note on #10225

While testing this manually against my own editable install, I ran into a separate, pre-existing bug: modulefinder crashes on marimo's own namespace packages, which silently breaks dependency detection, but only in dev setup, not for regular users. It briefly made this fix look broken even though it wasn't, but when I tested it with the a built app, it worked.

📋 Pre-Review Checklist

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✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jul 19, 2026 4:33pm

[Sanjays2402]

This separate map never gets cleared when a module name is rebound to another __file__. check() explicitly drops modules_mtimes on that path but leaves this old baseline behind, so a replacement file with an older mtime and subsequent edits stay invisible until its timestamp passes the previous file’s.

[Felipenguim]

52527f2
Now in the functioncheck() the watcher_modules_mtimes also gets cleared to deal with modules being rebound

f3dafec
A new test to ensure it