VS Code freezes when the last character of a custom event handler is deleted

[JBergles]

Describe the bug

I noticed some strange behavior when using a component with custom events. If I have <Foo on:bar={e => alert("hey")} /> and I start deleting from the end of the event handler, the VS code window hangs as soon as the body of the event handler becomes empty <Foo on:bar={} />

To Reproduce

Foo.svelte:

<script lang="ts">
    import { createEventDispatcher } from "svelte";
    const dispatch = createEventDispatcher();

    function func() {
        dispatch("bar", {abc: 123})
    }
</script>

App.svelte:

<script lang="ts">
	import Foo from "./Foo.svelte";
</script>

<Foo on:bar={e => alert("hey")} />

Place the cursor at the end of the event handler right before the closing }. Backspace until the event handler body becomes {}.

Expected behavior

VSCode doesn't crash and I can continue editing the app component.

System (please complete the following information):

• OS: Windows 10 Home 2004 19041.746
• IDE: VSCode 1.53.0 Remote on WSL2 (Ubuntu 20.04)
• Plugin/Package: Svelte for VSCode 104.4.0

Additional context

Minimal project to reproduce the issue
SvelteExample.tar.gz

[jasonlyu123]

This is weird. I can see a high CPU usage on the main process of vscode, not extension host or language server. From my understanding, the extension can't block the UI thread.

After some more digging, I found disabling svelte.plugin.typescript.semanticTokens.enable would prevent the problem. But I don't know why. It freezes before asking the language server to update semantic tokens.

[jasonlyu123]

Found the reason. There is a negative length in the semantic tokens result. We need to filter that out.