The default jQueryUI implementation was removed in version 8 because its weight is not always needed. But we are elft on our own to include jQueryUI, which gets used a lot, especially by component authors. The result is that many components will be loading jQueryUI, and often conflicting versions too.
The best solution would be to put a checkbox on each Web Block and Screen's properties for "Include jQueryUI". This way we have a consistent jQueryUI version number to code against, and do not need to manage/maintain/test this outselves.