How to know when an ItemsControl has overflowed its ScrollViewer?

Question

How would one swap a template by using a Trigger when the items within an ItemsControl overflow its container?

In my current implementation I would like to show a more brief form of template based on whether the current state of the ItemsControl are such that the content would cause a ScrollViwer's scroll bars to appear if the scroll bar visibility was set to Auto. So I'm interested in any property either on the ScrollViewer or the ItemsControl which may give me a clue about whether it is overflowing it's ScrollViewer. If I have to use some custom ValueConverter to make a sensible boolean value to bind to I guess that would be alright too.

Answer 1

Ok so I ended up figuring this one out myself.

In my case I'm using Logical Scrolling as opposed to physical scrolling and since I'm have a horizontal layout all I had to do was to trigger on the ScrollViewer.ScrollableWidth. The secret to getting this working in my case was to make sure that I set ScrollViewer.HorizontalScrollBarVisibility to Hidden as opposed to Disabled. This will alow the content to be scrolled and will make it so that the scrollableWidth property actually has a meaningful value.

NOTE: Curiously setting HorizontalScrollBarVisibility to Hidden seems to collapse the scroll bar visually so I'm not entirely sure what the difference is between Collapse and Hidden in this case.