9550 4k - extending desktop onto non-high DPI display

Hi,

I've just taken delivery of a 9550 with the 4k display. This is my first experience of a high DPI display in windows. When I'm in work, I extend my windows desktop onto a 24" HD monitor. So I have the laptop set to 225% scaling, and the 24" monitor set to 100% scaling. The issue is that when I first connect the external monitor, although the fonts are the expected size, they look fuzzy. If I designate the external monitor as the primary display & reboot, images then become sharp on the external monitor, but fuzzy on the laptop (although they remain scaled correctly). 

Similarly, if I then disconnect the external monitor, text remains fuzzy on the laptop until I reboot.

Essentially, although I can set scaling individually for each monitor, I can only have sharp text on 1 monitor at a time, and need to reboot following disconnecting/reconnecting the external monitor in order to get sharp text on the primary monitor.

Is this a windows thing, an intel driver thing, or a dell thing? Or none of the above?! Is there a solution?