Showing multi diff editor take 2 seconds

[isidorn]

Testing #199155

Clicking on the Multi Diff Editor until the content appears feels sluggish. On my machine it looks like 1s+, and you can clearly see the white screen.

Can we make this more performant? Or to at least use some tricks that we immediately start rendering

https://github.com/microsoft/vscode/assets/1926584/40c00507-7f60-49a8-8058-480c6a609e7f

[hediet]

I have an idea how we can improve this:

• We only render the first diff editors where we fully computed the diff (initially 0).
• We show the multi diff editor immediately, but show a loading spinner at the bottom until all diff editors computed their diffs.

Then there will be no flickering, as this only appends content.

[isidorn]

Sounds like a reasonable approach to me.