Capture iframe csp/sandbox errors

I'm writing Cypress tests to validate iframe security policies around sandbox & CSP permissions. I have disabled allow-modals and calls to alert and others fail as expected the following Chrome generated error message: VM271 about:srcdoc:1 Ignored call to 'alert()'. The document is sandboxed, and the 'allow-modals' keyword is not set. Great, as expected.

Now I need to build a Cypress test to prove that the behavior on the iframe is correct. I was initially going to do this by spying on the iframe/window's console.error and checking for a call with the above error message. This did not work. I have wrapped the window, iframe, and cypress environment error handlers and console.error functions with a spy and none of them are called when Chrome logs the above 'ignore alert' error message. I also wrapped the iframe window.alert() in a spy and it is still called (as expected) but the browser's alert modal doesn't show (as expected because it's being ignored by chrome).

Is there a way to catch the browser's 'special' error log here? If it's not being logged through the browser window or iframe window elements, how is it being logged?

Iframe:<iframe><script>alert('test');</script></iframe>

Cypress:

let iframeWindow;let spyErrorIframe;cy.get('iframe').then(($iframe) => {  iframeWindow = $iframe[0].contentWindow;  function errorHandler(...args) {    console.log('~~~ iframe');    return iframeWindow.console.error(...args);  }  spyErrorIframe = cy.spy(errorHandler);  iframeWindow.addEventListener('error', spyErrorIframe);  iframeWindow.onerror = spyErrorIframe;  cy.spy(iframeWindow.console, 'log');  cy.spy(iframeWindow.console, 'error');  cy.spy(iframeWindow, 'alert');});let mainWindow;let spyErrorWindow;cy.window().then((win) => {  mainWindow = win;  function errorHandler(...args) {    console.log('~~~ window');    return iframeWindow.console.error(...args);  }  spyErrorWindow = cy.spy(errorHandler);  mainWindow.addEventListener('error', spyErrorWindow);  mainWindow.onerror = spyErrorWindow;  cy.spy(win, 'alert');  cy.spy(win.console, 'log');  cy.spy(win.console, 'error');});cy.wait(GENERIC_TIMEOUT + 1000).then(() => {  console.log('~~~ iframe console.error', iframeWindow.console.error.getCalls()); // no calls  console.log('~~~ window console.error', mainWindow.console.error.getCalls()); // no calls  console.log('~~~ cypress console.error', console.error.getCalls()); // no calls  console.log('~~~ iframe error event', spyErrorIframe.getCalls()); // no calls  console.log('~~~ window error event', spyErrorWindow.getCalls()); // no calls  console.log('~~~ iframe alert', iframeWindow.alert.getCalls()); // one call as expected  console.log('~~~ window alert', mainWindow.alert.getCalls()); // no calls});