Allow the browser-test port to be set with TEST_PORT

CONTRIBUTING.md

@@ -33,7 +33,7 @@ npm ci
 npm test
 ```
 
-The tests start their own Python HTTP server on port 8897 and use installed Chrome by default. To use bundled Chromium instead:
+The tests start their own Python HTTP server on port 8897 (set `TEST_PORT` to change it) and use installed Chrome by default. To use bundled Chromium instead:
 
 ```sh
 npx playwright install chromium

tests/browser.test.cjs

@@ -4,11 +4,16 @@ const {chromium} = require('playwright');
 const {spawn} = require('node:child_process');
 const fs = require('node:fs');
 let browser, server;
-const base = 'http://127.0.0.1:8897/';
+const port = process.env.TEST_PORT || '8897';
+const base = `http://127.0.0.1:${port}/`;
 before(async()=>{
-  server=spawn(process.env.PYTHON || 'python3',['-m','http.server','8897','--bind','127.0.0.1'],{stdio:'ignore'});
+  server=spawn(process.env.PYTHON || 'python3',['-m','http.server',port,'--bind','127.0.0.1'],{stdio:'ignore'});
+  // A server that exits early usually means the port is taken; fail rather than test another server's files.
+  let exited=false;server.on('exit',()=>{exited=true;});
   // Consume the readiness response so its open stream does not outlive the hook.
   for(let i=0;i<50;i++){try{await (await fetch(base)).arrayBuffer();break;}catch{await new Promise(r=>setTimeout(r,100));}}
+  await new Promise(r=>setTimeout(r,200));
+  if(exited) throw new Error(`Test server could not start on port ${port}; set TEST_PORT to a free port.`);
   browser=await chromium.launch({channel:process.env.BROWSER_CHANNEL || 'chrome',headless:true});
 });
 after(async()=>{await browser?.close();server?.kill();});