Cleaner closing

[krish-vj]

PR Description: Fix Simulator Window Transition Hangs, C++ Object Deletion Crashes, and Windows Console Unicode Errors

Summary

This PR addresses several critical stability issues occurring when transitioning from the simulator/flowsheet workspace back to the landing page, as well as console encoding crashes on Windows platforms.

Identified Issues & Root Causes

1. "Not Responding" / Abrupt Freezes on Window Transition

• Root Cause: When closing the MainApp (flowsheet) window, the landing page previously called deleteLater() on the MainApp instance. Because MainApp owns complex sub-components, dock widgets, simulation threads, and scene items, calling deleteLater() schedules asynchronous C++ destruction. This destruction races with the Qt event loop. When the landing page restores, repaints, and handles focus, it triggers queries or events on objects whose C++ counterparts have already been deleted, resulting in Windows "Not Responding" hangs or abrupt segmentation faults.

2. Cursor Stack Pollution & Corrupted Interaction

• Root Cause: Override cursors (like WaitCursor or PointingHandCursor) set during simulation execution or drag-and-drop operations were not fully drained from Qt's cursor stack upon window closure. The polluted cursor stack caused mouse-tracking and hover events to fail on the restored Landing Page, making it seem locked or unresponsive.

3. Unicode Console Encoding Crashes on Windows

• Root Cause: Several print() statements in Container.py and undo_manager.py used the unicode right-arrow character →. On Windows machines running standard non-UTF-8 console codepages (e.g. CP1252), this caused uncaught UnicodeEncodeError exceptions, causing background routines to crash.

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Solutions & Implementation Details

1. Safe Window Stashing (No deleteLater())

• Modified Landing_page.py to hide the closed MainApp window and stash it in a module-level list _stashed_windows instead of deleting it. Keeping the window alive but hidden completely bypasses C++ destruction issues and races, with negligible memory impact.
• Swapped out unstable closeEvent monkey-patching wrapper with a standard PyQt closed signal connection.

2. Robust Thread & Cursor Cleanup

• Implemented a clean closeEvent in mainApp.py that:
  • Safely stops any running simulation thread (self.thrd).
  • Drains the QApplication override cursor stack.
  • Emits the closed signal and accepts the event.
• Added a cursor draining loop in the landing page's restore_landing_page method to ensure a clean cursor stack state before showing the landing page.
• Added try-except RuntimeError guards in dock widget cleanup to handle instances where underlying C++ objects may have been freed elsewhere.

3. ASCII Arrow Replacements for Console Prints

• Replaced all unicode → characters with ASCII -> in Container.py and undo_manager.py.
• Stripped BOM headers from modified files to prevent encoding errors on older Python interpreters.

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