Align (scrolling) TabControl with MD spec

[nicolaihenriksen]

Fixes #3976

This is a work in progress attempting to align the TabControl (scrolling) style with the MD spec.

The PR contains the following changes:

• Introduces TabControlHeaderScrollBehavior in order to react the tab/scroll events.
• Introduces custom StackPanel (currently named BringIntoViewHijackingStackPanel to signal intent) in order to hijack and re-raise the RequestBringIntoView event.
  • It sits in between the ScrollViewer and the contained TabItems. It "hijacks" the FrameworkElement.RequestBringIntoViewEvent by marking it as handled, and subsequently issue a new FrameworkElement.BringIntoView(Rect) call with a Rect matching the original event, but offset left/right based on scroll direction.
• Adds a "padding" to the left of the first tab, and to the right of the last tab as per the MD spec.
  • This padding is only applied if the width of the tabs overflow the available width.
• Adds smooth/animated scrolling behavior when a partially visible tab needs to be brought into view.
  • This is done by animating a custom AP which in turn calls ScrollViewer.ScrollToHorizontalOffset() - thanks @Keboo for that idea!
  • A slight hack is used to prevent users to click on anything while the (very short) animation is running.
• Adds TabAssist.TabScrollDuration AP to allow consumers to control the animation duration. TimeSpan.Zero effectively disables the animation.
  • Default value set in Style to allow easy override at the call site.
• Adds TabAssist.TabScrollOffset AP to allow consumers to modify the offset (i.e. "padding" from above).
  • Default value set in Style to allow easy override at the call site.
• 2 new UI tests which currently don't assert anything, but allow for easy testing of the feature.

I have done some preliminary testing and I think it works pretty well. However I don't have a mouse with a mouse-wheel-tilt feature, so that part lacks testing (I could imagine that may cause some issues). I have tried to avoid conflicts with it by setting a private variable (_desiredScrollStart) when the scrolling is initiated from a tab change. I hope that is enough, but only testing will show.

Things to look at before PR is ready:

• Remove debugging Debug.WriteLine statements.
• Ensure "mouse-wheel tilt" feature behaves nicely with this feature.
• Consider if BringIntoViewHijackingStackPanel is really the name we want for this panel.
• Ensure FlowDirection=RightToLeft works as expected.
• Write proper assertions in UI tests (or delete the tests if we cannot find proper assertions)
• Consider if the slight hack preventing user input while animating can be done in a better way.
• Other things?

Preview of the feature