Regression : Dialog is not getting the Foucs after minimized the parent & selecting the parent using ALT+TAB.

Step to Reproduce
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1) Run the attached program.
2) Click on the "Click Me" Button. You can see a dialog.
3) Click on "Minimise" button of parent frame. You can see both the parent frame and Dialog gets minimised.
4) Press ALT + TAB key you can see the ICONs . Select parent frame Icon(ModalDialogTest).
   You can see both the parent frame & dialog on the screen . If you dn't see the
   "Focus" on the Dialog then the bug is reproduce.

I tested this in Mustang b10 to b73 & it is reproducable.
The source of the problem is quite the same as in 6272225, so I'm closing this CR as duplicate. See comments for 6272225 for more details.