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When you have Assembly navigator undocked, it becomes transparent any time you
do not have the mouse on that window.
Solution
To disable the dialog transparency select:
File --> Preferences --> User Interface ---> Theme --> untoggle the Enable Transparency option.
Notes and References
Hardware/Software Configuration
Platform: na
OS: n/a
OS Version: n/a
Product: NX
Application: SYSENG
Version: V1847
Function: DISPLAY
Ref: 001-7868339