This PR fixes #80666 . I was really hoping that this will be fixed for September update, but no-surprisingly, was pushed back.

Disclaimer

Be aware that I was unable to test this in vscode build itself, because It's very problematic to get build running. I've tested similar code in debugger's conditional breakpoint and this code in simplified test scenario. But yeah, real-life test is necessary.

The problem

LineDecorations often are changing order when there is after decorator, which messes up drag&drop. With Error Lens extension this is annoying as... something very annoying. More info #80666 (comment)

The solution

There are 2 approach for this: first to sort every LineDecoration array, but I've settled for second: make comparison order-proof. For starters changes are done in one place, but also future changes will less likely break this.
I've tried to optimize this as good as I could, but maybe this may be improved further.

The elephant

Isn't order important somehow?
No. Which style is used should be determinated by order of CSS rules, so even in different order, decorators should show-up in the same way. If this wasn't the case, this would cause weird style flashing.

The performance

For arr.length <= 1 this could be even faster. For arrays in correct order above 2 elements, this will have overhead of additional array (bSeen), but still should be the same number of comparisons (w/ _equals). On the other hand usually there isn't that many visible elements with 2+ decorators.

However for arrays with different order this will also prevent re-rendering these lines - which happens more often than you may think.

@alexdima This is probably for you to take a look. Or maybe @rebornix ?