Visual state manager VisualStateGroups null instead of crashed visual state

[marcelwgn]

Describe the bug

The progressbar template of this commit does not have a resource for high contrast. When switching from "normal" to high contrast, instead of crashing, the VisualStateManager gives us null for the VisualStateGroups.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Check out the linked commit
2. Start app
3. Switch to high contrast while app is open
4. Visit ProgressBar page

Expected behavior

VisualStateManager should not return null for the visual state groups.
Screenshots

Version Info

NuGet package version:

Windows app type:

UWP	Win32

Windows 10 version	Saw the problem?
Insider Build (xxxxx)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Xbox
Surface Hub
IoT

Additional context

Here is the DLM file for the error/crash:

HighContrast-Crash.zip

[ranjeshj]

@stevenbrix @evelynwu-msft as FYI

[JeanRoca]

@chingucoding thanks for filing this. Would you be able to clarify what the expected behavior is here? A repro would also be useful if possible.

[marcelwgn]

The issue is that in high contrast , for some reason, the visual state manager return null for the list of visual state groups despite the same call returns something differently on other themes. See @StephenLPeters comment for more context.

I hope that this app repros the issue (it did not repro on my machine back then iirc so I can't verify it will now):
VSMCrash.zip

[stevenbrix]

@chingucoding, VSM is designed somewhat like {Binding}, where it is supposed to silently fail. It sounds like you're asking that we change this behavior, correct?

[marcelwgn]

I suppose, yes. But the VSM can also crash when switching to states with invalid references which is why this behavior was confusing to see.

[stevenbrix]

Oh geese, does the repro you provided have that scenario? It would be helpful to have that scenario. Inconsistent behavior is worse than consistently incorrect behavior.

[marcelwgn]

IIRC, yes the repro was exactly that case where in high contrast (I think), we failed to look up a resource but instead of crashing we just got an empty list of visual states.