[Enhancement] Tab switch behavior after upspliting a tab

[sporocyst]

In 1.9b, when you upspliting a tab using the split handle on top of page, you will always be kept at the page you upsplitted and leave the prior splitted tabs. This behavior shoud be modified.
For example, if I have splitted tabs and try to upsplit tab that I am 'not' working on, it should just kick out the upsplited page from my current sight and I would stay at these splitted tabs. Otherwise, if I upsplit the page I 'am' working on, I should end up staying at same page while seeing it not in splitted tabs anymore, which is in accord with current default behavior.

In short, the tab you stay at should be the one that is selected/activated before upsplitting a tab, not the upsplitted tab.