Properly restore focus when TabControl.SelectedIndex is set via DialogHost

[nicolaihenriksen]

Fixes #3094

As per the discussion thread below, this PR adds DialogHost.RestoreFocusElement and DialogHost.IsRestoreFocusDisabled attached/dependency properties.

The former is currently used for the TabItem style (also the one for the "rail item") to ensure focus is returned to the parent TabControl rather than the TabItem itself.

The latter is used to completely disable the restore focus feature in case that is relevant.

Added 4 UI tests to verify the behavior.

[Keboo]

After giving it some thought here would be my proposal.

1. Create a new DialogHost.RestoreFocusElement (perhaps with a better name)
2. When setting DialogHost._restoreFocusDialogClose first check that element to see if the above attached property has a value, if it does use it for the _restoreFocusDialogClose value.
3. In the style for the TabItem (MaterialDesignTabItem) we do a relative binding looking for a parent tab control (something like: {Binding RelativeSource={RelativeSource FindAncestor, AncestorType=TabControl}}). We may need to look at other styles as well, such as MaterialDesignNavigationRailTabItem.
4. Optionally, we could consider a mechanism for turning off the restore focus behavior (I say optional because rarely has anyone ever requested this). But we could create a static property on DialogHost (something like DialogHost.DisableRestoreFocus or a better name). Then in step 2 above if that is the value of the attached property, we would set DialogHost._restoreFocusDialogClose to null to effectively disable the behavior.

Thoughts?

[nicolaihenriksen]

After giving it some thought here would be my proposal.
...
Thoughts?

I think this sound like a very good approach. I very much like the idea that the responsibility for picking the correct element to focus is removed from the DialogHost and put into the respective controls themselves. This aligns nicely with the Tell Don't Ask principle. Also, it is probably a rather limited number of controls which face this type of (possible) issue.

Having the additional option to completely disable the restore behavior also seems like a good idea to me. We'll just default the property to false such that it is something a caller would have to explicitly request for the DialogHost.

I have added the properties discussed and UI tests to test the scenarios. What remains is agreeing on the names of the properties, and applying the attached property on other controls suffering from the same issue (e.g. MaterialDesignNavigationRailTabItem)

[Keboo]

This is great. I think there is a bit of a disjoint on the names of the properties. (FocusRestore vs RestoreFocus). I think IsFocusRestoreDisabled should probably be the one changed to IsRestoreFocusDisabled. Thoughts?

[nicolaihenriksen]

This is great. I think there is a bit of a disjoint on the names of the properties. (FocusRestore vs RestoreFocus). I think IsFocusRestoreDisabled should probably be the one changed to IsRestoreFocusDisabled. Thoughts?

I agree, I'll change it.

[VR462002]

Hi Nicolai and Kevin,

thank you for working on this, you guys are awesome!

@nicolaihenriksen Thanks for testing and explaining the focus problem!
@Keboo Always appreciate your input and enjoy your live videos/recordings