[release/8.0-preview7] [Blazor] Improvements to the interaction between SSR and interactive rendering

[MackinnonBuck]

Improvements to the interaction between SSR and interactive rendering

This PR improves how Blazor's new SSR feature blends with interactive component rendering.

Description

Before this PR, full-stack Blazor had multiple limitations when using server-side rendering (SSR) and interactive rendering (Blazor Server/WebAssembly components) in combination:

• Interactive components could not be added after the initial page load
• Enhanced navigation was disabled after the first interactive component was added
  • This meant that a full page reload would occur when the route changed, even for "internal" navigations

Note: From now on, I'm going to refer to updates to static content that don't involve a page reload as "SSR updates". This includes enhanced navigations and stream rendering updates.

This PR enhances the experience by:

• Allowing interactive components to be added via SSR updates
• Allowing enhanced navigations to occur even after interactive components exist
• Persisting component instances across SSR updates
  • i.e., user-entered state does not get destroyed when an SSR update occurs
• Enabling an interactive component's parameters to get changed by an SSR update
• Correctly disposing interactive component instances when an SSR update removes them

In summary, these changes make the boundary between SSR and interactive rendering much more seamless than it was before.

Fixes #48763
Fixes #48764

Customer Impact

Medium/high. Many of the issues we've run into when doing internal app building are due to the aforementioned limitations. This is a change we're anticipating will greatly increase the capabilities of apps customers can build, so it's critical we get feedback about whether the behavior of this feature meets their needs.

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

This is a large change that touches some fairly fundamental parts of the framework. Existing automated tests help give us reasonable confidence that existing Blazor WebAssembly and Server applications (non-SSR'd) won't be affected by this change. However, the possibility of behavioral changes in certain edge cases is non-zero.
New automated tests validate the functionality of the enhancements this PR brings.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[ghost]

Hi @MackinnonBuck. 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.

[mkArtakMSFT]

Approving given that the original PR was approved and this is a backport only.