https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/AutoCAD-Viewcube-and-Navigation-bar-not-diplayed-in-some-viewports.html
Issue:
In some viewports, the viewcube and navigation bar are not displayed even when NAVBAR is set to 1.
Causes:
This occurs with viewports that were copied using the COPY command.
Viewports that were copied via the clipboard (e.g. using COPYCLIP > PASTECLIP or CTRL+C > CTRL+V) are not affected.
Solution:
To get the navigation controls to display in an affected viewport:
- Double-click inside the viewport to activate it
- Type navswheel and press ENTER
- Click outside the viewport to deactivate it
- Double-click again inside the viewport to activate it.
The navigation controls should now display.
An alternative method is to change the visual style of the viewport (and change it back, if desired).
AutoCAD Viewcube and Navigation bar not diplayed in some viewports
原文：https://www.cnblogs.com/heroius/p/13191391.html
