At the top of the source windows in VS 2017 is a toolbar with drop-downs for code browsing. I have discovered that using the following steps the toolbar disappears until the file is closed and re-opened.
If the context menu is used to “peek” a definition that is in the same file, when the peek window is closed the toolbar goes away.
Sorry, this is a known bug caused by the internals of the Peek Definition logic and fully resolving it would involve a fairly large change on the VisualGDB side. As a workaround please try reopening the source file after the bug is triggered.