Enhance Blazor hosting model configuration content

[guardrex]

Fixes #15373

Internal Review Topic (links to section)

😕 ... I've run into a point of confusion that's even blocking this PR ...

Our content implies (and I was under the assumption) that when autostart="false" is placed on the script that calling Blazor.start() is absolutely required to get the app going. Testing here seems to indicate that that's not so. When I set the _Host.cshtml page up with only the following ...

<script autostart="false" src="_framework/blazor.server.js"></script>

and no call to Blazor.start(), the app still starts and runs normally, which I don't expect to happen.

What am I missing? ..... I'll adjust the PR accordingly to head off the (my 😄) confusion.

[captainsafia]

@guardrex Content here looks good.

I wasn't able to reproduce the issue you cited. My repro steps are as follows:

1. Create new project via dotnet new blazorserver -o AutostartTest
2. Modify Pages/_Host.cshtml to set autostart='false'.
3. Observe that the circuit isn't initialized.

This was on 5.0.100-rc.1.20431.3.

Is there a chance that you have a Blazor.start invocation somewhere else in the code?

[guardrex]

"Circuit" ... I expected to see the warning bar that a circuit hasn't been established. Turns out that there's no indication ... just everything that would depend on the circuit no-ops. Therefore, I'd like to look again at the text here. It should probably say somehow explicitly that "start" in Blazor.start and autostart refers to 'starting the circuit' and perhaps that only functionality that depends on the circuit is impacted by a disabled circuit ... something like that ... I'll work on the wording. I'll update it tomorrow (Tuesday) morning and ping u back.

[guardrex]

@captainsafia ... Here's the latest opening remarks of that section ...

Configure the manual start of a Blazor Server app's [SignalR circuit](xref:blazor/hosting-models#circuits) in the `Pages/_Host.cshtml` file:

• Add an `autostart="false"` attribute to the `<script>` tag for the `blazor.server.js` script.
• Place a script that calls `Blazor.start` after the `blazor.server.js` script's tag and inside the closing `</body>` tag.

When `autostart` is disabled, any aspect of the app that doesn't depend on the circuit works normally. For example, client-side routing is operational. However, any aspect that depends on the circuit isn't operational until `Blazor.start` is called. For example, component methods fail to execute while the circuit is disconnected.

[guardrex]

@captainsafia ... does this look good ☝️?

[captainsafia]

Nice additions! Left some feedback inline.

[guardrex]

@captainsafia ... Confirm the CSS ... is this correct ...

#components-reconnect-modal {
    transition: visibility 0s linear 500ms;
}

Nevermind ... got it ... looks correct based on dotnet/aspnetcore#24566.