Hi,
This specific combination is currently problematic. The issue here is that the TPlannerMaskDatePicker is making its dropdown the active window while changing the active window triggers that the dropdown closes. There isn't an easy solution for this (as you'd still need to handle changing of active window to close the dropdown when you'd click outside the dropdown). We'll need to investigate if this can be something technically be handled.
ok, I don't need to specifically use TPlannerMaskDatePicker. I need some kind of DateTimePicker which is able to handle somehow an empty field (not using 01.01.1899). Can you suggest one of yours in the component pack?
The other controls that have a dropdown part to pick a value from will remain problematic as this dropdown changes the active window.