OK it got a bit further, then got stuck at:
Traceback (most recent call last):
File "../../../../../src/3rdparty/chromium/third_party/node/node.py", line 63, in <module>
RunNode(sys.argv[1:])
File "../../../../../src/3rdparty/chromium/third_party/node/node.py", line 58, in RunNode
raise RuntimeError('%s failed: %s' % (cmd, stderr))
RuntimeError: ['C:/USR/LOCAL/BIN/node.exe', '../../../../../src/3rdparty/chromium/third_party/devtools-frontend/src/node_modules/rollup/dist/bin/rollup', '--silent', '--plugin', 'terser', '--config', '../../../../../src/3rdparty/chromium/third_party/devtools-frontend/src/front_end/rollup.config.js', '--input', 'gen/third_party/devtools-frontend/src/front_end/formatter_worker/FormatterActions.prebundle.js', '--file', 'gen/third_party/devtools-frontend/src/front_end/formatter_worker/FormatterActions.js'] failed: [!] (plugin terser) TypeError: Cannot read property 'length' of undefined
TypeError: Cannot read property 'length' of undefined
at new JestWorker (U:\dev\qt6-webengine-os2\src\3rdparty\chromium\third_party\devtools-frontend\src\node_modules\jest-worker\build\index.js:131:39)
at Object.renderChunk (U:\dev\qt6-webengine-os2\src\3rdparty\chromium\third_party\devtools-frontend\src\node_modules\rollup-plugin-terser\rollup-plugin-terser.js:22:23)
at Promise.resolve.then (U:\dev\qt6-webengine-os2\src\3rdparty\chromium\third_party\devtools-frontend\src\node_modules\rollup\dist\shared\rollup.js:18988:25)