Double-clicking to create new tab creates two when Sublime not in focus

[iamed2]

When not in focus, double-clicking on the end of the tab strip to create a new tab opens two new tabs instead. This has happened on every build of Sublime Text 3 that I recall, on macOS.

[wbond]

This is being tracked on the bug tracker at https://github.com/SublimeTextIssues/Core/issues/1600 and https://github.com/SublimeTextIssues/Core/issues/2210.