To be more verbose:
1. Open Privacy Browser.
2. Tap the + to add a new tab. Leave it blank.
3. Move back to tab 1.
4. Scroll the app bar off the screen.
5. Open the navigation menu by swiping from the left.
6. Close tab 1.
7. The empty tab 2 becomes the only tab. There is no way to scroll the app bar back onto the screen.
This is caused because, when the WebView is empty, there is no way scroll it down, so no scrolling action gets passed onto the app bar.
As a side note, there are two ways to recover from this without restarting Privacy Browser, both of which are unacceptable as a solution to the problem.
1. Open any website via bookmarks or the home page. It will then be possible to scroll the app bar back into view.
2. If you started with more than two tabs, you can still switch to another tab by swiping left or right in the ViewPager. Once you reach a tab that has content, you can scroll the app bar back into view.
Because of the way Android handles the hiding of the app bar, it might be difficult to teach it to scroll the app bar when the WebView is not populated. It might be easier to simply display the app bar programmatically whenever a tab is deleted.