grabGesture doesn't work when a QScrollArea is inside a QDialog and the dialog is opened by calling exec()

grabGesture doesn't work when a QScrollArea is inside a QDialog and the dialog is opened by calling exec(). The problem doesn't occur if the dialog is opened by calling show().

Steps to reproduce:
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• run the sample attached

Phase 1:

• use the mouse left button to grab the widget inside the scroll area
• move/throw the widget around
• press the button "Open a new dialog" -> Dialog with an identical scroll widget opens

Phase 2:

• use the mouse to move the widget inside the newly opened dialog
• observe the behavior and compare it to 'Phase 1'
  EXPECTED: the widget behaves the same way
  ACTUAL: the widget cannot be moved by grabbing