Laptop screen black (but flashes to life when opening new browser tab?)

W10, HP ProBook 4540s.

Upon startup this morning, HP logo was flickering/glitching. For about 3sec, the right-half of the display was rainbow/striped, and the left half was black. Then complete darkness.

Everything boots and I'm posting from the machine now. External monitor by HDMI port. VGA also works, so it's not graphics card related. I suspect the ribbon cable is worn, but only replaced it 2yrs ago.

What's strange is that the full screen comes to life briefly (just a full-screen flicker) when I open a new Browser page (Brave), and when I open a new tab in Brave. The content displays.

What's double-strange is that this behavior does not happen at all with Firefox.

Any guesses/tips, before I crack it open?

Thank you!