Setting tab order in WPF

You can skip elements in the tab sequence by setting KeyboardNavigation.IsTabStop on the element in XAML.

KeyboardNavigation.IsTabStop="False"

You can setup a trigger that would toggle this property based on the expanded state.

If you want to explicitly set the tab ordering for elements in your form, the following attached property is supposed to help:

<Control KeyboardNavigation.TabIndex="0" ... />

I say "supposed to help" as I haven't found it very reliable though I probably need to read more about how it is intended to be used. I only post this half baked answer because no one else mentioned this property.

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Note that in Win RT, the property is just TabIndex="0".