Random screen flickering on water-damaged 2017 13" MacBook Pro

My computer has recently started "flickering". Randomly but frequently a black screen will replace the normal display. This black screen lasts just a fraction of a second, but can happen a couple of times a second. Sometimes there are longer intervals between these flashes. But the flickering happens so often that the system is unusable. I've reset the SMC and reset NVRAM, neither having an effect on the flickering. Somewhat surprising, though: the problem completely disappears if I connect an external monitor. Both displays are normal with no anomalies.

I did spill water on the keyboard a year or two ago. When that happened I immediately turned the system upside down and kept it that way for a day. So it's possible there's some latent water damage, but other than the screen flickering -- which is a fairly recent development -- the Macbook has worked fine.