feat: highlight hovered nodes

packages/editor/src/lib/Workspace.svelte.ts

@@ -1,13 +1,13 @@
 import type { CompileError, CompileResult } from 'svelte/compiler';
-import { Compartment, EditorState } from '@codemirror/state';
+import { Compartment, EditorState, StateEffect, StateField } from '@codemirror/state';
 import { compile_file } from './compile-worker';
 import { BROWSER } from 'esm-env';
 import { basicSetup, EditorView } from 'codemirror';
 import { javascript } from '@codemirror/lang-javascript';
 import { html } from '@codemirror/lang-html';
 import { svelte } from '@replit/codemirror-lang-svelte';
 import { autocomplete_for_svelte } from '@sveltejs/site-kit/codemirror';
-import { keymap } from '@codemirror/view';
+import { Decoration, keymap, type DecorationSet } from '@codemirror/view';
 import { acceptCompletion } from '@codemirror/autocomplete';
 import { indentWithTab } from '@codemirror/commands';
 import { indentUnit } from '@codemirror/language';
@@ -51,6 +51,32 @@ function file_type(file: Item) {
 	return file.name.split('.').pop();
 }
 
+const set_highlight = StateEffect.define<{ start: number; end: number } | null>();
+
+const highlight_field = StateField.define<DecorationSet>({
+	create() {
+		return Decoration.none;
+	},
+	update(highlights, tr) {
+		// Apply the effect
+		for (let effect of tr.effects) {
+			if (effect.is(set_highlight)) {
+				if (effect.value) {
+					const { start, end } = effect.value;
+					const deco = Decoration.mark({ class: 'highlight' }).range(start, end);
+					return Decoration.set([deco]);
+				} else {
+					// Clear highlight
+					return Decoration.none;
+				}
+			}
+		}
+		// Map decorations for document changes
+		return highlights.map(tr.changes);
+	},
+	provide: (field) => EditorView.decorations.from(field)
+});
+
 const tab_behaviour = new Compartment();
 const vim_mode = new Compartment();
 
@@ -60,7 +86,8 @@ const default_extensions = [
 	tab_behaviour.of(keymap.of([{ key: 'Tab', run: acceptCompletion }])),
 	indentUnit.of('\t'),
 	theme,
-	vim_mode.of([])
+	vim_mode.of([]),
+	highlight_field
 ];
 
 export interface ExposedCompilerOptions {
@@ -225,6 +252,14 @@ export class Workspace {
 		});
 	}
 
+	highlight_range(node: { start: number; end: number } | null) {
+		if (!this.#view) return;
+
+		this.#view.dispatch({
+			effects: set_highlight.of(node)
+		});
+	}
+
 	mark_saved() {
 		this.modified = {};
 	}

packages/editor/src/lib/codemirror.css

@@ -304,4 +304,9 @@
 			}
 		}
 	}
+
+	.highlight {
+		background: hsl(60, 100%, 80%);
+		padding: 4px 0;
+	}
 }

packages/repl/src/lib/Output/AstNode.svelte

@@ -12,9 +12,10 @@
 		path_nodes?: Ast[];
 		autoscroll?: boolean;
 		depth?: number;
+		onhover: (node: { type: string; start: number; end: number } | null) => void;
 	}
 
-	let { key = '', value, path_nodes = [], autoscroll = true, depth = 0 }: Props = $props();
+	let { key = '', value, path_nodes = [], autoscroll = true, onhover, depth = 0 }: Props = $props();
 
 	const { toggleable } = get_repl_context();
 
@@ -97,7 +98,14 @@
 			{/if}
 		</span>
 	{:else}
-		<details bind:open>
+		<!-- svelte-ignore a11y_mouse_events_have_key_events (seems like a false positive) -->
+		<details
+			bind:open
+			onfocusin={() => onhover(value)}
+			onfocusout={(e) => onhover(null)}
+			onmouseover={() => onhover(value)}
+			onmouseleave={() => onhover(null)}
+		>
 			<summary>
 				{#if key}
 					<span class="key">{key}</span>:
@@ -123,6 +131,7 @@
 						value={v}
 						{path_nodes}
 						{autoscroll}
+						{onhover}
 						depth={depth + 1}
 					/>
 				{/each}

packages/repl/src/lib/Output/AstView.svelte

@@ -65,7 +65,19 @@
 		<code>
 			{#if typeof ast === 'object'}
 				<ul>
-					<AstNode value={ast} {path_nodes} {autoscroll} />
+					<AstNode
+						value={ast}
+						{path_nodes}
+						{autoscroll}
+						onhover={(node) => {
+							if (
+								node === null ||
+								(node.type !== undefined && node.start !== undefined && node.end !== undefined)
+							) {
+								workspace.highlight_range(node);
+							}
+						}}
+					/>
 				</ul>
 			{:else}
 				<p>No AST available</p>