Editor hang/poor performance with a large number of tests in a file

[DanTup]

I'm investigating some performance issues in Dart when you have a large number of tests in a single file. Many of these issues are in the Dart extension, but while working on it I've been seeing a few bad hands in the editor that I think may be a VS Code issue. It seems to happen during syncDecorations which I suspect are related to the test icons in the gutter.

The test file I'm using has approximately 2,500 tests in the file.

Here's an export of the profile from the dev tools:

slow_decorations.json.zip

Note: I can't give precise repro steps using the Dart extension right now because I'm still making performance improvements (if you try to reproduce this with the published version of the extension you'll see significant performance issues in the extension that I'm working on), but if this profile isn't helpful on its own and you need more, I may be in a position to publish a pre-release version with some of my fixes soon.

@connor4312 FYI - I don't know if this is general decorations code or test-specific, but I think the anonymous function showing up a lot in the profile relates to this code here:

vscode/src/vs/workbench/contrib/testing/browser/testingDecorations.ts

Line 199 in 2377c7a

const map = model.changeDecorations(accessor => {

[connor4312]

Repro steps:

1. Use the test provider sample

node -p "new Array(2500).fill(0).map((a, i) => \`1 + \${i} = \${i + 1}\`).join('\n')" > sample/big.md

3. F5, open big.md, and click on a decoration

[DanTup]

@connor4312 seems much better now (in Insiders), thank you!