This is not a help request. I just thought that I would post this here for others.

When you have EnableTabScroll="true" and adding tabs does not seem to give you scroll settings.... make sure that the tabs are not just resizing to a smaller width value and allowing the new tab to appear without scrolling becoming necessary.

I spent too long today failing to notice that nothing was actually wrong.

Same goes for when you resize the window on what looks like a full set of tabs. You expect the scrolling to appear but it may not be necessary.