fix: adds headless ternary to handle 'new', true, and false in a non-breaking fashion (#1485) (#1486) (#1511)

Puppet and Playwright complain and pollute logs with warnings about "new headless mode". This reads the value set in backstop.json, or uses true if 'headless' is not set, or 'new', as a string. README updated with details.

Closes: #1485

Author: dgrebb

README.md

@@ -644,12 +644,13 @@ ignoreHTTPSErrors: true,
 headless: <!!!config.debugWindow>
 ```
 
-You can add more settings (or override the defaults) with the engineOptions property. (properties are merged)
+You can add more settings (or override the defaults) with the engineOptions property. (properties are merged). This is where headless mode can also be set to 'new', until "new headless mode" is the default in Puppet/Playwright.
 
 ```json
 "engineOptions": {
   "ignoreHTTPSErrors": false,
   "args": ["--no-sandbox", "--disable-setuid-sandbox"],
+  "headless": "new",
   "gotoParameters": { "waitUntil": "networkidle0" },
 }
 ```

core/util/runPlaywright.js

@@ -36,7 +36,7 @@ module.exports.createPlaywrightBrowser = async function (config) {
   const playwrightArgs = Object.assign(
     {},
     {
-      headless: !config.debugWindow
+      headless: config.debugWindow ? false : config?.engineOptions?.headless || true
     },
     config.engineOptions
   );

core/util/runPuppet.js

@@ -71,7 +71,7 @@ async function processScenarioView (scenario, variantOrScenarioLabelSafe, scenar
     {},
     {
       ignoreHTTPSErrors: true,
-      headless: !config.debugWindow
+      headless: config.debugWindow ? false : config?.engineOptions?.headless || true
     },
     config.engineOptions
   );