tests: add Playwright E2E tests with screenshot golden testing (+ fix examples session handling) (#115)

* Add Playwright E2E tests with screenshot golden testing

- Add E2E tests for all 8 MCP server examples
- Screenshot golden images for visual regression testing
- CI workflow for running E2E tests
- npm scripts: test:e2e, test:e2e:update, test:e2e:ui

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Co-Authored-By: Claude <noreply@anthropic.com>

* Add default demo code for Three.js example

When no code is provided, show a rotating green cube demo.

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* Add E2E testing documentation to CONTRIBUTING.md

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* Update Three.js golden screenshot with 3D cube

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* Add explicit permissions to CI workflow

Set minimal read-only permissions for security best practices.

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* Fix test.setTimeout() to be inside describe blocks

Playwright requires setTimeout to be called within a test or describe block.

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* Simplify E2E tests - remove excessive timeouts

- Remove explicit setTimeout calls (default 30s is sufficient)
- Replace waitForTimeout(5000/6000) with proper waitForAppLoad()
- Wait for inner iframe visibility instead of fixed delays
- Keep only 500ms stabilization for screenshot animations

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* Auto-generate missing snapshots in CI

Use updateSnapshots: 'missing' in CI to handle cross-platform
screenshot differences (macOS vs Linux).

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* Use platform-agnostic golden screenshots

- Remove -chromium-darwin suffix from snapshot filenames
- Configure snapshotPathTemplate for cross-platform compatibility
- Increase tolerance to 5% for rendering differences between platforms

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* Refactor tests to use forEach instead of for-of loop

May help with Playwright test discovery issues in CI.

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* Use npm ci and list reporter in CI

- Use npm ci for exact package versions
- Use --reporter=list instead of html to avoid potential re-evaluation issues
- Only upload test-results on failure

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* Exclude e2e tests from bun test

Run bun test only on src/ directory to avoid running Playwright tests
with Bun's test runner.

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* Fix prettier formatting

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* Add interaction tests for basic server apps

Test that clicking buttons in the MCP App triggers the corresponding
host callbacks:
- Send Message → host logs "Message from MCP App"
- Send Log → host logs "Log message from MCP App"
- Open Link → host logs "Open link request"

Tests both React and Vanilla JS implementations.

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* Mask dynamic content in E2E screenshot tests

Add Playwright mask option to handle servers with dynamic/random content:
- basic-react/vanillajs: mask server time display
- system-monitor: mask CPU chart, memory stats, uptime
- cohort-heatmap: mask heatmap grid (random data)
- customer-segmentation: mask scatter chart (random data)

This addresses PR feedback about handling examples with non-deterministic
output. Masking replaces the previous 10% tolerance with proper exclusion
of dynamic elements, allowing tighter 1% tolerance for the rest of the UI.

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* Add wiki-explorer to E2E tests with default URL param

- Add default URL "https://en.wikipedia.org/wiki/Model_Context_Protocol" to
  wiki-explorer-server's get-first-degree-links tool inputSchema
- Update basic-host to automatically populate input field with tool defaults
  extracted from inputSchema.properties
- Add wiki-explorer-server to E2E test suite with dynamic masking for the
  force-directed graph

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* Use .default() for threejs tool schema instead of .optional()

This exposes the default code and height values in the JSON Schema,
allowing basic-host to auto-populate the input field with defaults.

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* Enable parallel E2E tests with timeouts and canvas masking

- Enable fullyParallel with 4 workers locally (2 in CI)
- Add 30s timeout per test
- Mask threejs canvas for stable screenshots
- Update snapshots to reflect default values in input fields

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* Add pre-commit check for private registry URLs in package-lock.json

Same check as in CI to catch issues before push.

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* Increase screenshot diff tolerance to 6% for cross-platform rendering

Font rendering differs between macOS and Linux, causing ~5% pixel differences.

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* Revert pre-commit artifactory check (moved to separate PR #133)

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* Format threejs server.ts

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* Update CONTRIBUTING.md to reflect platform-agnostic screenshots

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* fix(e2e): use factory pattern for MCP servers to support parallel connections

McpServer only supports one transport at a time. When multiple browser
contexts connected in parallel, calling server.connect(transport) for
each session overwrote the previous transport's callbacks, causing
connection corruption and test timeouts.

Changes:
- server-utils.ts: accept factory function instead of single instance
- All 9 example servers: wrap server creation in createServer() function
- playwright.config.ts: re-enable parallel execution (4 workers)
- servers.spec.ts: add toBeEnabled wait for server connection
- Update screenshot baselines for basic-vanillajs and cohort-heatmap

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---------

Co-authored-by: Claude <noreply@anthropic.com>

examples/basic-host/src/index.tsx

@@ -1,9 +1,30 @@
+import type { Tool } from "@modelcontextprotocol/sdk/types.js";
 import { Component, type ErrorInfo, type ReactNode, StrictMode, Suspense, use, useEffect, useMemo, useRef, useState } from "react";
 import { createRoot } from "react-dom/client";
 import { callTool, connectToServer, hasAppHtml, initializeApp, loadSandboxProxy, log, newAppBridge, type ServerInfo, type ToolCallInfo } from "./implementation";
 import styles from "./index.module.css";
 
 
+/**
+ * Extract default values from a tool's JSON Schema inputSchema.
+ * Returns a formatted JSON string with defaults, or "{}" if none found.
+ */
+function getToolDefaults(tool: Tool | undefined): string {
+  if (!tool?.inputSchema?.properties) return "{}";
+
+  const defaults: Record<string, unknown> = {};
+  for (const [key, prop] of Object.entries(tool.inputSchema.properties)) {
+    if (prop && typeof prop === "object" && "default" in prop) {
+      defaults[key] = prop.default;
+    }
+  }
+
+  return Object.keys(defaults).length > 0
+    ? JSON.stringify(defaults, null, 2)
+    : "{}";
+}
+
+
 // Host passes serversPromise to CallToolPanel
 interface HostProps {
   serversPromise: Promise<ServerInfo[]>;
@@ -74,6 +95,14 @@ function CallToolPanel({ serversPromise, addToolCall }: CallToolPanelProps) {
     setSelectedServer(server);
     const [firstTool] = server.tools.keys();
     setSelectedTool(firstTool ?? "");
+    // Set input JSON to tool defaults (if any)
+    setInputJson(getToolDefaults(server.tools.get(firstTool ?? "")));
+  };
+
+  const handleToolSelect = (toolName: string) => {
+    setSelectedTool(toolName);
+    // Set input JSON to tool defaults (if any)
+    setInputJson(getToolDefaults(selectedServer?.tools.get(toolName)));
   };
 
   const handleSubmit = () => {
@@ -96,7 +125,7 @@ function CallToolPanel({ serversPromise, addToolCall }: CallToolPanelProps) {
           <select
             className={styles.toolSelect}
             value={selectedTool}
-            onChange={(e) => setSelectedTool(e.target.value)}
+            onChange={(e) => handleToolSelect(e.target.value)}
           >
             {selectedServer && toolNames.map((name) => (
               <option key={name} value={name}>{name}</option>