fix an issue with sheet closing animation - the following steps were showing an anomaly:   - open Widgetsgallery -> Sheets -> Sheet -> Systemwide sheet   - select Auto-focus and Chained checkboxes   - Open systemwide sheet   - close the sheet => you can see a glitch during this closing animation
fix an issue with sheet closing animation
- the following steps were showing an anomaly:
  - open Widgetsgallery -> Sheets -> Sheet -> Systemwide sheet
  - select Auto-focus and Chained checkboxes
  - Open systemwide sheet
  - close the sheet
=> you can see a glitch during this closing animation

This was due to the behind window was not properly set up for the animation.

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