I've seen this in the past, but can't remember the cause. My guess would be to update the video driver.
I looked at my copy of TurboCAD 2018 Expert (Win10) and don't have this problem in either GDI or RedSDK (Options / Native Draw).
Thanks for the reply, I've tried a few variants of the Radeon HD driver but nothing resolved the problem. FWIW, I have this problem on two machines, an i7 Dell XPS desktop with AMD HD 6770, and a Dell 7000 series laptop with dual video cards, Integrated Intel and a discrete Nvidia card. Some of the drivers that I tested left me with only GDI as the Native Draw option in the 2017 TurboCAD and the problem persists in 2018 version, so it doesn't seem like GDI or RedSDK is the issue. Because I have the dual cards on the laptop I've been able to test lots of hardware options and nothing has worked. I'm sure it will have a common cause if I can find it, perhaps it's related to Win 10 Update 1809.
Interesting I don't have RedSDK in Expert 2018, only GDI, but I do have RedSDK in the Expert 2017 version.
My motivation to get this all working again is to upgrade to Pro Platinum, but I don't want to commit to that if I also need new hardware. But it seems odd Expert 2017 works with RedSDK and Expert 2018 will only use GDI yet has these workspace issues.
FWIW, I have also tested with and without OpenGL Hardware Acceleration enabled.