Launch QA browsers on SwiftShader where there is no Metal

The shared launcher always asked for ANGLE's Metal backend, which exists
only on macOS; on CI's Linux runners WebGL failed and the Street Level gate
never saw the app start. The default is now Metal on macOS and SwiftShader
elsewhere; QA_ANGLE still overrides it.

scripts/qa-strict-harness.mjs

@@ -27,7 +27,12 @@ import {
   STRICT_TILE_HOSTS,
 } from './qa-map-policy.mjs';
 import { createMissLoader, tileCacheFile } from './qa/tileCache.mjs';
-import { closeBrowser, diagnoseExit, exitAfterFlush } from './qa/browser.mjs';
+import {
+  closeBrowser,
+  diagnoseExit,
+  exitAfterFlush,
+  qaAngle,
+} from './qa/browser.mjs';
 import { DAYTIME, shiftTime } from './qa/time.mjs';
 import {
   PLANESPOTTERS_EMPTY_BODY,
@@ -78,7 +83,7 @@ export const RESERVED_PORTS = Object.freeze([
     .split(',').map((port) => port.trim()).filter(Boolean),
 ]);
 /** Chrome flags every scenario's browser starts with (the GPU path). */
-export const CHROME_ARGS = Object.freeze([`--use-angle=${process.env.QA_ANGLE || 'metal'}`, '--enable-gpu']);
+export const CHROME_ARGS = Object.freeze([`--use-angle=${qaAngle()}`, '--enable-gpu']);
 
 /** The values of --layers=a,b and --evidence=FILE (null when absent). */
 export function strictArguments(argv = process.argv) {

scripts/qa/browser.mjs

@@ -1,6 +1,14 @@
 import { processTree, readProcesses } from './process.mjs';
 import puppeteer from 'puppeteer';
 import { chromeExecutablePath } from '../qa-chrome.mjs';
+/**
+ * The ANGLE backend: QA_ANGLE, else Metal on macOS and SwiftShader elsewhere
+ * (CI's Linux runners have no GPU, and Metal does not exist there).
+ */
+export function qaAngle(env = process.env, platform = process.platform) {
+  return env.QA_ANGLE || (platform === 'darwin' ? 'metal' : 'swiftshader');
+}
+
 /** Keep every scenario on the same headless GPU path. */
 export function launchBrowser({ args = [], ...options } = {}) {
   return puppeteer.launch({
@@ -8,7 +16,7 @@ export function launchBrowser({ args = [], ...options } = {}) {
     ...options,
     headless: true,
     args: [
-      `--use-angle=${process.env.QA_ANGLE || 'metal'}`,
+      `--use-angle=${qaAngle()}`,
       '--enable-gpu',
       ...args,
     ],

scripts/qa/browser.test.mjs

@@ -1,7 +1,7 @@
 import assert from 'node:assert/strict';
 import test from 'node:test';
 import puppeteer from 'puppeteer';
-import { launchBrowser } from './browser.mjs';
+import { launchBrowser, qaAngle } from './browser.mjs';
 
 test('scenario launch uses the selected ANGLE backend and stays headless', async (t) => {
   const prior = process.env.QA_ANGLE;
@@ -35,5 +35,11 @@ test('scenario launch uses the selected ANGLE backend and stays headless', async
   assert.equal(options.protocolTimeout, 120000);
   delete process.env.QA_ANGLE;
   await launchBrowser();
-  assert.equal(options.args[0], '--use-angle=metal');
+  assert.equal(options.args[0], `--use-angle=${qaAngle({})}`);
+});
+
+test('the default backend is Metal on macOS and SwiftShader elsewhere', () => {
+  assert.equal(qaAngle({}, 'darwin'), 'metal');
+  assert.equal(qaAngle({}, 'linux'), 'swiftshader');
+  assert.equal(qaAngle({ QA_ANGLE: 'vulkan' }, 'linux'), 'vulkan');
 });