docs: update with focus jumping fix completion

Window split functionality is now fully stable with proper focus management.
All major issues resolved:
- No crashes or freezing during splits
- Proper text rendering in all windows
- Stable focus that doesn't jump during typing
- Working cursor movement in focused windows
- Proper modeline display for each window

Split windows are now production-ready for use.

DEV_STATE.md

@@ -72,6 +72,7 @@ Lumacs/
 - ✅ **Split Window UI Polish**: Fixed modeline display, focus stability, and split ratio calculations
 - ✅ **Window Split Freeze Fix**: Resolved GTK signal_realize callback issues causing freezes during splits
 - ✅ **Split Window Cursor Fix**: Fixed cursor movement to work in focused window rather than original active window
+- ✅ **Focus Jumping Fix**: Resolved spurious next_window() calls causing focus to jump during typing
 
 ## Todo  
 1. **Phase 7**: Performance optimization and tuning