Feature/optimistic UI quest actions

[mijinummi]

Summary

This PR improves perceived application responsiveness by introducing optimistic UI updates for user actions that previously waited for full server round-trips before updating the interface. The implementation applies immediate client-side state updates, gracefully rolls back changes when requests fail, and preserves consistency with the server once requests complete.

In addition to the optimistic update implementation, this work includes performance validation, regression coverage, and documentation to ensure the improvement is measurable, maintainable, and safe for future development.

Closes #2055

Problem

Previously, user-triggered mutations waited for server confirmation before the UI reflected any changes. Although functionally correct, this resulted in:

• Noticeable delays before the interface updated
• Reduced perceived responsiveness under high network latency
• Additional user friction during repeated interactions
• Poorer overall user experience despite successful backend processing

The lack of optimistic rendering made common actions feel slower than necessary.

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Solution

Implemented optimistic UI updates across applicable mutation flows using a predictable optimistic-update lifecycle.

The implementation:

• Applies immediate local state updates before server confirmation
• Preserves previous state for rollback
• Restores the previous state automatically if a request fails
• Synchronizes with the server after successful completion
• Keeps cache consistency across affected queries
• Prevents stale UI state after mutation completion

This approach significantly improves perceived performance while maintaining data integrity.

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Implementation Details

Optimistic Mutation Workflow

Implemented an optimistic update lifecycle consisting of:

1. Cancel in-flight queries affecting the same resource
2. Snapshot existing cache/state
3. Apply optimistic UI update immediately
4. Execute server mutation
5. Roll back to previous state on failure
6. Invalidate/refetch affected queries after completion

This ensures the UI remains responsive without sacrificing correctness.

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Rollback Support

Added deterministic rollback handling that:

• Restores cached state after failed mutations
• Prevents inconsistent client state
• Displays server-validated data after synchronization
• Handles network failures gracefully

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Cache Synchronization

Improved cache management by:

• Updating affected query caches optimistically
• Invalidating stale data after mutation completion
• Preventing duplicate fetches where unnecessary
• Preserving consistency across related views

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Performance Improvements

The updated workflow reduces perceived interaction latency by eliminating unnecessary waiting before rendering user actions.

Performance validation included:

• Lighthouse
• React Profiler
• Bundle analysis (where applicable)

Measurements demonstrate faster UI feedback while preserving existing functionality.

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Testing

Expanded regression coverage to verify:

• Successful optimistic updates
• Rollback behavior on mutation failures
• Cache consistency
• Successful synchronization after server responses
• Existing application behavior remains unchanged

All existing test suites continue to pass.

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Documentation

Updated project documentation to include:

• Optimistic update architecture
• Mutation lifecycle
• Rollback strategy
• Cache synchronization flow
• Guidelines for implementing future optimistic mutations

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Performance Validation

Before

• UI updated only after server response
• Noticeable interaction latency
• Lower perceived responsiveness
• Additional waiting under slower network conditions

After

• Immediate UI feedback
• Reduced perceived latency
• Faster interaction experience
• Automatic rollback when requests fail
• Consistent synchronization with server state

Performance measurements were validated using Lighthouse, React Profiler, and project profiling tools where applicable.

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Files Updated

• Implemented optimistic mutation handling
• Added rollback support
• Updated cache synchronization logic
• Added regression tests
• Updated developer documentation

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Acceptance Criteria

• ✅ Implemented optimistic updates for applicable user actions
• ✅ Added rollback support for failed mutations
• ✅ Improved perceived UI responsiveness
• ✅ Verified measurable performance improvements
• ✅ Preserved existing functionality
• ✅ Added regression tests
• ✅ Updated project documentation
• ✅ All tests pass
• ✅ Code follows project standards

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Testing Performed

• Verified optimistic rendering for successful mutations
• Verified rollback restores previous state after failures
• Verified cache invalidation and synchronization
• Verified no regressions in existing workflows
• Verified all automated tests pass
• Verified application behavior under network failure scenarios
• Verified consistent UI state across repeated mutations

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Impact

This change significantly improves the perceived responsiveness of the application by reducing the time users wait for visual feedback after performing actions. The implementation maintains server consistency through rollback and cache synchronization, resulting in a faster, more resilient, and more user-friendly experience without altering existing business logic.

[RUKAYAT-CODER]

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