Same issue here. I may have identified the source of the glitch.
If I play with the designer tool on those low graphic settings the game gives me at a new lot, the designer tool window opens right where I clicked on screen (and where the edited object is), the cursor changes to an arrow icon and everything works well.
After I update my graphic settings to 4k resolution, high details etc., the designer window either opens at a completely random location (while my cursor is still the designer tool icon that I used to select the object; it doesn't allow me to do anything except going into the designer window, then it changes) or doesn't open at all – with the game still running, but not allowing me to do anything while my cursor being the designer tool icon).
I guess the designer window actually opens somewhere off-screen so we can't move the mouse into it to change the cursor and do anything.
