Steps to Reproduce

1. shrink the window size to make the tabbox scrollable

Current Result

the selected tab is not in the view

Expected Result

the selected tab is visible in the view

Debug Information

• tabbox moves the selected tab into the current view is the built-in behavior. We can verify this behavior by shrinking the window width again or increase.
• When a tabbox is scrollable (show left and right arrow icon), if you shrink or increase the window width, the tabbox always move the selected tab into the current view.
• it's better to refactor _scrollcheck() into several readable functions.