I'm using xfce4 and my theme is orchis-dark. I was always wondering about a select button on the left corner of my window.
You can see the button in the following screenshots:
What does it do?
Ask Ubuntu is a question and answer site for Ubuntu users and developers. It only takes a minute to sign up.Sign up to join this community
I can't be 100% sure, because you are using a non-default theme, but chances are that this is the Stick button.
What this button does is that it makes the window "sticky" so it would appear on all workspaces.
You can make sure that this is the case by opening Settings Manager → Window Manager → Style tab and hover over this button in the Button layout section. You should get a tooltip as shown in the following screenshot:
Sidenote: you can also hide and rearrange the position of the buttons in your windows title bar by dragging and dropping them in the respective frames of the Button layout section.