@victordcq already explained the issue you’re encountering, as did this thread: Perspective Odd dateTimeInput Behavior
In the example you provided, attempting to open the “regular background” DTI results in an expanded picker which ends up expanding below the originating component and you’ll notice it also extends further to the left than the originating component. The “black background” DTI has no space to the left, and so the expanded picker enters the “modal” mode. If you want to avoid the modal mode, your originating DTI will need to be at least as wide as the picker which will be expanded. The picker is 252px wide, so your DTI must be 252px wide in order to avoid modal mode.
Please note that you’ll also encounter modal mode if your originating DTI is close enough to right edge of the viewport.