Use formatWithCursor() to preserve cursor position

[ntotten]

Summary

• Use Prettier's formatWithCursor() API instead of format() to preserve cursor position after formatting
• The cursor no longer jumps to an arbitrary position when formatting a document
• Properly handles edge cases like range formatting (where cursor position doesn't apply) and older Prettier versions

Changes

• types.ts: Added PrettierCursorOptions and PrettierCursorResult type exports, and added formatWithCursor method to PrettierInstance and PrettierModule interfaces
• PrettierDynamicInstance.ts: Implemented formatWithCursor method that delegates to the underlying Prettier module
• ModuleResolverWeb.ts: Added formatWithCursor support for the browser/web extension
• PrettierEditService.ts:
  • Updated format() to optionally use formatWithCursor() when a cursor offset is available
  • Updated provideEdits() to capture cursor position before formatting and schedule cursor repositioning after VS Code applies the edit
  • Skips cursor handling for range formatting where cursor preservation doesn't make sense

Test plan

• Open a file with some unformatted code
• Place cursor somewhere in the middle of the file
• Format the document (Cmd/Ctrl+Shift+I or format on save)
• Verify cursor position is preserved at the equivalent location in the formatted output
• Test range formatting still works correctly (cursor handling is skipped)
• Test in browser/web extension environment

Fixes #3939

Generated with Claude Code

[Copilot]

Pull request overview

This PR implements cursor position preservation during document formatting by utilizing Prettier's formatWithCursor() API instead of the regular format() method. This addresses issue #3939 where the cursor would jump to an arbitrary position after formatting.

Changes:

• Added formatWithCursor method to Prettier type interfaces and implementations
• Updated the formatting service to capture cursor position before formatting and restore it afterward
• Properly handles edge cases like range formatting where cursor preservation doesn't apply

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

File	Description
src/types.ts	Added PrettierCursorOptions and PrettierCursorResult type exports; added formatWithCursor method to PrettierInstance and PrettierModule interfaces
src/PrettierDynamicInstance.ts	Implemented formatWithCursor method that delegates to the underlying Prettier module
src/ModuleResolverWeb.ts	Added formatWithCursor support for browser/web extension
src/PrettierEditService.ts	Updated formatting logic to use formatWithCursor() when cursor offset is available; added cursor repositioning logic using setImmediate

[Copilot]

The setImmediate function is not available in browser environments, but this extension supports both Node.js and browser contexts (as indicated by the "browser" field in package.json). This will cause a runtime error when the extension runs in vscode.dev or other browser-based VS Code instances.

Consider using setTimeout(() => {...}, 0) instead, which works in both Node.js and browser environments and provides similar behavior of deferring execution to the next event loop tick.

[Copilot]

The new cursor position preservation feature lacks test coverage. Consider adding a test that:

1. Opens a document with unformatted code
2. Places the cursor at a specific position
3. Triggers formatting
4. Verifies the cursor is at the expected position in the formatted output

This is especially important since this feature involves timing-sensitive operations with setImmediate and the interaction between VS Code's edit application and cursor repositioning.