fix(web): prevent task checkboxes from overwriting large Markdown files

[yaacovcorcos]

What Changed

• Large Markdown files still open as rendered previews, but their task-list checkboxes are now read-only.
• The preview also has a defensive guard that refuses task-list updates whenever the file contents are truncated.
• The existing size warning now says explicitly that the preview is read-only.
• Regression tests cover both a truncated file larger than the 1 MiB preview limit and an ordinary complete Markdown file.

Why

The file reader intentionally returns only the first 1 MiB of a large text file and marks that result as truncated. The rendered Markdown path was selected before the general read-only truncated-text path, however, so task-list checkboxes still had an active save handler.

Clicking one of those checkboxes could send the truncated prefix to the whole-file writer. That could replace the complete Markdown file with only its first 1 MiB and discard everything after the preview limit.

This change keeps the useful rendered preview while removing its mutation path. Complete Markdown files behave exactly as before.

UI Changes

The existing warning now begins with “Read-only preview,” and task checkboxes in a truncated Markdown preview no longer accept changes. The layout is unchanged.

Visual proof

This screenshot is from an isolated desktop build of this PR using a synthetic 1,232,144-byte Markdown file. After clicking the visible checkbox, it remained disabled and unchecked. The file also retained its original SHA-256 (f96f00c1a67f7aa39888fbdacf85f6075e033ed3e6f6946b80fe1710c076559e), byte count, and END-OF-FILE-SENTINEL-MUST-SURVIVE tail marker.

Validation

• pnpm exec vp test run apps/web/src/components/files/FilePreviewPanel.test.ts apps/web/src/components/files/fileSaveCoordinator.test.ts apps/web/src/components/files/projectFilesQueryState.test.ts — 12 tests passed
• pnpm --filter @t3tools/web typecheck
• targeted lint for the three changed files
• targeted format check for the three changed files
• git diff --check

Checklist

• This PR is small and focused
• I explained what changed and why
• I included visual proof for the UI change
• A video is not applicable; this change removes an unsafe interaction and has no animation

Model: GPT-5. Harness: OpenAI Codex.

Note

Prevent task checkboxes from modifying truncated Markdown files in file preview

• Adds resolveMarkdownTaskPreviewUpdate in filePreviewMode.ts that returns null for truncated content or no-op changes, replacing direct use of setMarkdownTaskChecked.
• Disables the onTaskListChange handler in RenderedMarkdownSurface when the file read is truncated, preventing partial overwrites of large files.
• Updates the truncated file banner copy to "Read-only preview" to signal non-interactivity.

^{Macroscope summarized 56d80e6.}

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Note

Medium Risk
Touches the Markdown preview save path that writes project files; a missed truncated guard could still cause data loss, though the change itself is a narrow defensive fix.

Overview
Prevents truncated Markdown previews from saving partial content when task-list checkboxes are toggled.

Rendered Markdown still opens for large files, but onTaskListChange is disabled when the read is truncated. A new resolveMarkdownTaskPreviewUpdate helper also refuses updates for truncated content as a second guard. The size warning now says Read-only preview. Complete Markdown files keep the previous editable checkbox behavior.

^{Reviewed by Cursor Bugbot for commit 56d80e6. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[macroscopeapp]

Approvability

Verdict: Approved 56d80e6

Straightforward bug fix that prevents data loss by disabling task checkbox interactions when viewing truncated Markdown files. The change is simple, self-contained, and includes appropriate tests.

^{You can customize Macroscope's approvability policy. Learn more.}