fix(watch): skip reload when notebook has git conflict markers

[mscolnick]

Summary

When marimo edit --watch is running and a git operation (cherry-pick,
git stash pop, merge) writes conflict markers into the notebook file,
the watcher parses the file and the conflict markers become an
app._unparsable_cell. This clobbers the on-disk conflict markers
through the round-trip and breaks conflict-resolution tools like
git-mediate.

The fix: before reloading, scan the file for lines starting with
<<<<<<< (git's unambiguous conflict start marker). If present, log a
warning and skip the reload until the user resolves the conflicts.

Fixes #9613.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 21, 2026 6:41pm

[Copilot]

Pull request overview

Prevents marimo edit --watch from clobbering on-disk git conflict markers by detecting merge-conflict start markers in the notebook file and skipping reloads until conflicts are resolved (fixes #9613).

Changes:

• Add a pre-reload scan for git conflict markers (<<<<<<< at start of a line) and skip reload with a warning when present.
• Refactor “skip our own writes” logic to reuse a single file read via a new content_matches_last_save() helper.
• Add a regression test ensuring reload is skipped and a warning is emitted when conflict markers exist.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/_session/test_file_change_handler.py	Adds a regression test asserting watch reload is skipped when conflict markers are present.
marimo/_session/notebook/file_manager.py	Introduces content_matches_last_save() and reuses it from file_content_matches_last_save().
marimo/_session/file_change_handler.py	Reads file once for pre-reload checks; skips reload on conflict markers and on own writes.

[cubic-dev-ai]

No issues found across 3 files

Architecture diagram

sequenceDiagram
    participant FS as File System
    participant Watch as File Watcher
    participant Handler as FileChangeHandler
    participant FM as AppFileManager
    participant Session as Session

    Note over FS,Session: File change detection and reload flow

    FS->>Watch: File modified event
    Watch->>Handler: handle_change(file_path, session)
    
    Handler->>FM: read_file()
    FM-->>Handler: current_content (str)
    
    alt Content matches last save (our own write)
        Handler->>Handler: Return FileChangeResult(handled=False)
        Note over Handler: Skip reload to avoid echo loop
    else Content has git conflict markers (<<<<<<<)
        Handler->>Handler: Log warning
        Handler->>Handler: Return FileChangeResult(handled=False)
        Note over Handler: NEW: Skip reload to preserve conflict markers
    else Normal content change
        Handler->>FM: reload() with current_content
        FM->>FM: Parse notebook, apply changes
        alt Parsable notebook
            FM-->>Handler: Success
            Handler->>Session: Apply cell changes
            Session-->>Handler: OK
            Handler-->>Watch: FileChangeResult(handled=True)
        else Unparsable notebook
            FM->>FM: Create _unparsable_cell
            FM-->>Handler: Partial/error
            Handler-->>Watch: FileChangeResult with error
        end
    end

    Note over FS,Handler: Error handling path
    alt read_file() fails
        Handler->>Handler: Log debug, set current_content=None
        Handler->>Handler: Fall through to reload()
        Note over Handler: reload() has its own error handling
    end

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[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.23.7-dev82