On two machines I got this odd behaviour today.
On saving and reopening the file (without closing rhino) it behaves normal again:
Rhino 6 SR3 2018-3-31 (Rhino 6, 6.3.18090.471, Git hash:master @ b32d93f452fce970e96deb46bfc68b075f06b290)
Windows 10.0 SR0.0 or greater (Physical RAM: 24Gb)
GeForce GTX 970/PCIe/SSE2 (OpenGL ver:4.6.0 NVIDIA 388.13)
OpenGL Settings
Safe mode: Off
Use accelerated hardware modes: On
Redraw scene when viewports are exposed: On
Anti-alias mode: 8x
Mip Map Filtering: Linear
Anisotropic Filtering Mode: Height
Vendor Name: NVIDIA Corporation
Render version: 4.6
Shading Language: 4.60 NVIDIA
Driver Date: 10-27-2017
Driver Version: 23.21.13.8813
Maximum Texture size: 16384 x 16384
Z-Buffer depth: 24 bits
Maximum Viewport size: 16384 x 16384
Total Video Memory: 4 GB