Question: I am having an issue in Visual Studio 2015 (VB.Net) where the Navigation Bar is not showing.

I have set the settings in Tools > Options > Text Editor > All Languages and set the "Navigation Bar" setting to checked.

The bar will show up for a second and then disappear.

I have tried it in Safe Mode and still the same.

I have tried editing the CurrentSettings.vssettings and it shows when you load but again it then disappears.

Any Thoughts?

Answer: 
Try this: **uncheck** the checkbox in tools\options\text editor\all languages\general\navigation bar and click OK. This is unintuitive, but then--come back to the same checkbox, which should now be Unchecked, check it, and click OK. This has worked for me. Why the navigation bar disappears randomly is a mystery.
for me such issue happens in the scenario that in one file the navigation bar disappeared but in other files it is still there. my solution is:
1) find the project that the missing navigation bar file located;
2) unload the project(in solution explorer->right click project name->in popup dialog, select unload project). Logically then the file will be closed and the project will be gray;
3) load the project again(in solution explorer->right click project name->in popup dialog, select load project).
Then the navigation bar will appear again in this file.
Such solution also help for Intelisense doesn't work for some files.