[Blazor] Hot reload testing improvements

[javiercn]

We have a long tail of hot reload issues that keep coming back up. As an effort to improve our team velocity we want to invest on some automation that allows us to validate a concrete set of scenarios on a regular basis. The goal of this automation is to act as a canary for when a new issue arises. We will run it on a daily basis as opposed to running it on every CI run.

The general structure for one of these tests is as follows:

• Starts with a pre-configured existing project.
• Runs dotnet watch or equivalent gesture.
• Loads a page in the browser using Playwright.
• Programmatically applies a change to one or more files (add a file, remove a file, edit its contents).
• Perform any action if needed (like instructing the browser to navigate to a new location)
• Validates that the changes have taken effect.

The list of scenarios is something that we will define, but includes things like:

• Adding a new component
• Adding a parameter to an existing component.
• Modifying the generated UI code.

[codymullins]

Consider adding tests around CSS changes (e.g., Tailwind compiling and replacing your app.css), please.

[thirstyape]

Will these tests be used to monitor performance metrics?

I know there are already other issues on the topic, mostly closed, but it seems like the performance of dotnet watch run went down the drain going from SDK 8.0 to SDK 9.0. The speed and reliability are both a fraction of what they were. Not to mention a new tab pops open every time.

[RobDeVoer]

Any change to html, css or less pages seens to trigger an error and requires a stop and restart. It is just starting to become unbearable to do any UI work. Wondering if I really need to restart hundreds of times just to update my UI design. Pages/Components are mostly generated content so can't do the updates in VS code and transport the changes.

[thirstyape]

Not sure what changed, but going from VS 17.12 LTSC to VS 17.14 LTSC has seemed to resolve whatever the issue was with hot reload.

Would still be nice to have a higher degree of assurance that future version upgrades will not cause problems.