Hidden title icon when vertical tabs is on

Just out of curiosity, I click on the far left of the title bar and found out the title icon is hidden underneath it. When double click it, it does what it always does: closing the window.

Not sure this is a bug or feature as I kind of want the legacy way of closing the window. Note that the title icon is unclickable when vertical tab is off. 

https://youtu.be/jihmPvfudzk Warning: Loud Sound at the beginning