[Blazor] Emit action attribute when not explicit.

[javiercn]

This change updates the server rendering logic to always emit an absolute URL for a form action when none is specified explicitly.

Description

When as part of rendering a form, the developer does not explicitly add an action attribute, the framework will automatically generate one with the value of the current request URL.

Fixes #51118

Customer Impact

When enhanced navigation is active, the URL might change while an update to the page is in progress. In such situations, if the user clicks a button and submits a form, the form might incorrectly post to the wrong URL, as the page URL might have already been updated.

Given that when the URL updates on the document is not a behavior, we have control over, we have to account for this, and to prevent it, we make sure that we always generate forms with an action attribute, which is unambiguous.

Regression?

• Yes
• No

[If yes, specify the version the behavior has regressed from]

Risk

• High
• Medium
• Low

The fix is to detect forms without an action attribute and emit it to the current URL. This is correct in all cases and is unambiguous.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

---

When servicing release/2.1

• Make necessary changes in eng/PatchConfig.props

[SteveSandersonMS]

Since you've confirmed you'll deal with the duplicate history entries thing, I'm happy to approve now and leave it with you to address the final bits.

[ghost]

Hi @javiercn. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

To learn more about how to prepare a servicing PR click here.

[ghost]

Hi @javiercn. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.