If for the DlgExpressionInput the user config parameter 'NoSystemBsckground' is set to True it's not used as a normal dialog but as a pop-up window.
In this mode the context-menu of the ExpressionLineEdit isn't working. A previous commit (2f4cec5e8a5d) fixed a crash but not this issue.
For Qt4 DlgExpressionInput has overridden the eventFilter() to handle using the context-menu but this is not working any more for Qt5 or Qt6.
Luckily, it has appeared that with Qt5 and Qt6 it works out-of-the box now and the event filter is not needed any more.
+ set application-wide event filter and handle mouse press events
+ fix expressPosition() to return exact position of line edit
+ fix warnings in ui file
+ use DlgExpressionInput as non-modal dialog in QuantitySpinBox