Added CornerRadius to DialogHost

[nicolaihenriksen]

Fix for #2772

Introduces a CornerRadius DP on DialogHost which is then used to round the corners of the PART_ContentCoverGrid (ie. the grid which shows a dimmed overlay covering the area that should be disabled while the dialog is open).

In order to support a CornerRadius, I introduced a Border. Ideally I guess the PART_ContentCoverGrid could have been changed to a Border but that could potentially break back-compat in case someone is doing their own template and providing a Grid named PART_ContentCoverGrid. To (hopefully) avoid introducing a breaking change, I simple placed a Border inside of the PART_ContentCoverGrid, and then use the OpacityMask on the PART_ContentCoverGrid to enforce the masking out of content that is not within the bounds of the contained Border.

My initial approach was to wrap PART_ContentCoverGrid in a Border, and the set ClipToBounds on the Border, but that does not work; the PART_ContentCoverGrid is still allowed to render its background in the corner area which is "trimmed away" by the Border, and thus it basically has no effect.

I added a simple UI test just to verify that the DP is correctly applied to the newly introduced Border (named ContentCoverBorder).

UPDATE I did not actually update the demo application to showcase this, because I didn't really think it made a lot of sense in the uses that are showcased. But to test it out, you can just set the DP on any of the DialogHost instances in there.

[Keboo]

This looks great. Would you mind adding one more UI test that uses the embedded dialog host style? Just to ensure that both styles leverage the property.

[nicolaihenriksen]

This looks great. Would you mind adding one more UI test that uses the embedded dialog host style? Just to ensure that both styles leverage the property.

Sure, no problem 👍