Create a document with a couple of headers, then show the Navigator (F5), then click on one of those headers in the navigator. Result: a floating "N.." toolbar appears containing two buttons: < and >, with tooltips "Back" and "Forward". I don't know why it does that, and I already have the ability to navigate by double-clicking headers so I'd rather not see it popping up an additional toolbar for this. I'm running the GTK3 version, if this changes anything.
This Navigation toolbar allows you to return easily to the previous cursor position. It is a complementary mean to navigate in your document. Note that you can anchor this toolbar at the end of the standard toolbar. So this behavior is not a bug, it's a feature ;-) I am closing it as Not a Bug. Please feel free to reopen if you disagree. In that case please add arguments. Best regards. JBF