My MacBook Pro screen is flickering, while external displays look fine

for your hardware issue— take it in

Yes the external display comes direct from the GPU. Your issue is the internal Display/Display Assembly.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"

Genius Bar Reservation and Apple Support Options - Apple

Outside the USA

Contact Apple Support - Apple Support

Get your mac repaired and how much will it cost

Mac Repair and Service - Apple Support

Get your Mac ready for service - Apple Support Get your Mac ready for service - Apple Support

For your Popup issue with mcAfee—

ref Block pop-up ads and windows in Safari

Block pop-up ads and windows in Safari - Apple Support -Apple Support

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

more—

Typically unwanted browser redirects are most always the result of something you installed

ref: Removing "Search Marquis" / "Search Baron" / etc on your own — John Galt

Removing "Search Marquis" / "Search Baron… - Apple Community

MrHoffman—docs/Doc: Scam Pop-ups, Scam Viruses, Scam Receipts, Scam Purchase

Scam Pop-ups, Scam Viruses, Scam Receipts… - Apple Community

Completely remove all McAfee components using the developers' instructions and extreme prejudice. Restart the computer and test.

Even If McAfee is not the cause, it can be masking the real culprit. The only benefits any Mac user get from overly-hyped anti-virus packages are:

• loss of performance
• loss of stability
• convoluted, hard to solve issues
• loss of money

If the eviction and restart do not clear the issue, then this is not a fix-at-home problem. Have Apple evaluate.

That looks like a hardware problem to me. Contact Apple and arrange for service.

Genius Bar Reservation and Apple Support Options - Apple