Solutions for Undo/Redo Buffer Loss on Format-on-Save

[joshuambg]

Reference Issue: #31021 - Redo buffer is cleared when saving a file

Problem Summary

Currently, format_on_save pushes a new undo state that clears the redo buffer, making it impossible to redo previously undone changes after saving. While @notpeter mentioned this would require "non-linear undo/redo history," I believe there are more targeted solutions that could preserve the current linear model while fixing this UX issue.

Proposed Solutions

Solution 1: Transparent Formatting Operations

The core issue isn't the undo/redo system itself—it's that format_on_save is being treated as a user action when it should be transparent.

Approach: Instead of pushing formatting changes as a new undo state, apply formatting while preserving the existing redo buffer by updating stored operations to account for formatting changes.

Implementation concept:

1. When format_on_save triggers, calculate the formatting diff
2. Transform existing redo operations by adjusting their positions based on formatting changes
3. Apply formatting without clearing the redo buffer

This is similar to operational transformation used in collaborative editors and is a well-solved problem for position-preserving changes like indentation and whitespace normalization.

Solution 2: Formatting Layers

Treat formatting as a separate layer that gets applied on save/display but isn't part of the core document state for undo/redo purposes.

Benefits:

• Formatting is always applied when needed (save, display)
• Content changes remain independent of formatting changes
• Users can still undo formatting by undoing to pre-format states
• Redo buffer remains intact for content modifications

Solution 3: Conditional Undo State Creation

Only push a new undo state for formatting if the formatting actually changed something substantial (beyond whitespace/indentation).

Implementation:

• Analyze formatting diff impact
• For whitespace-only changes: transform existing states instead of creating new ones
• For structural changes: create new undo state as current behavior

Expected User Experience

With any of these solutions:

1. User writes content → saves (with formatting) → undoes content → saves
2. User can still redo their original content changes
3. Formatting remains consistent throughout the process
4. No "lost forever" scenarios for content changes

The goal is maintaining the intuitive expectation that content changes remain redoable even after save operations, while keeping the elegance of the current linear undo/redo model.

[notpeter]

I've added a feature request for this.
Please 👍 upvote and subscribe to track progress.

• Support redo after autoformat changes buffer #31984