MacBook Pro display issue troubleshooting

Scott-he-him wrote:

Is it possible to initiate screen mirroring during startup or after using a key combination?

Yesterday when I turned on my MacBook Pro the screen was displaying vertical bars of varying color and width. Now the screen is all white with one small grey or off-white vertical bar. I do not have a physical connection type or monitor available to confirm the screen is the issue rather than the GPU or CPU.

Does anyone know if I could use any key combination to start screen mirroring to my Apple TV before sign-in or if a wired connection to my iPhone could act as an external display?

Thank you.

GPU or display assembly either way it is a hardware issue...

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"—

https://www.apple.com/retail/geniusbar/

Outside the USA

https://locate.apple.com/country

Learn how to get your Mac fixed and how much it will cost. https://support.apple.com/mac/repair/service

Get your Mac ready for service - Apple Support https://support.apple.com/en-us/HT209095

The keyboard shortcut:

Command-BrightnessDown

toggles the setting of : [√] Mirror Displays

-- this works for a display or TV set on a cable.

AppleTV is not strictly-speaking 'mirroring' and does not respond to this command -- it requires a more complex setup.

Have grey vertical bar about 2 inches wide on right side of screen Mac Pro purchased last year