MudAutocomplete: Coerce value immediately

[vernou]

Description

Before this PR, the value is coerced after the search. But the search can be delayed, which also delays the coercing of value :

After this PR, the value is coerced immediately :

Fixes #9420

How Has This Been Tested?

I added test.
I checked the doc page

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[ScarletKuro]

If you will work on this, pls update your branch as we added nullable annotation to this component

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.24%. Comparing base (aefe1e7) to head (06f303d).
Report is 19 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #10138   +/-   ##
=======================================
  Coverage   91.24%   91.24%           
=======================================
  Files         411      411           
  Lines       12495    12495           
  Branches     2434     2434           
=======================================
  Hits        11401    11401           
  Misses        552      552           
  Partials      542      542           

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[vernou]

If you will work on this, pls update your branch as we added nullable annotation to this component

Done.

---

As this PR has breaking change, I want do more test before.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[henon]

As this PR has breaking change, I want do more test before.

@vernou Do you still want to do more tests or can this be merged?

[vernou]

@vernou No, I finish to test. This is ready to be merged.

[henon]

If this fixes #9420 and that is considered a bug then I would say that this is not a breaking change per se.

[vernou]

@henon , the PR modify the behavior. It fixes the bug is a side effect.

---

The initial problem in #9420 was :

The value is coerced only if the search return 0 element, so the value is coerced after search, so after the debounce interval. But some event like OnBlur stop the debounce timer.

I see two solutions :

1. When Immediate is enable, then the value is coerced immediately. Would be the expected behavior, but it's a breaking change.
2. When the debounce timer is stoped, the value is coerced. Keep the weird behavior, but no breaking change.

As the modification will be integrated in v8, it's the good time to implement the solution 1.

[vernou]

Ping @henon

[henon]

I agree that it is a good time to do solution 1, adding this to the v8 migration guide. Thanks @vernou

[mikes-gh]

My IDE complains

The typeof operator cannot be used on a nullable reference type. (CS8639)

[ScarletKuro]

Yeah, that's a bug dotnet/razor#8692. Razor generator breaks when you use ? with generic in razor syntax, which is why I added CS8669 as NoWarn. Seems like it a similar warring. But I think in that case you can revert the T="string?" to T="string" as the MudAutocomplete already specifies the value as T? rather than adding a new NoWarn.
It's pretty sad to see that the razor plays not really nicely with nullable