Display arrange setting arbitrarily changing

External monitor is connected through HDMI & adapter to Macbook Air Thunderbolt port.

I set it to extend to the right of the laptop screen

Several times I have had to go back to Settings and re-do this because the mouse pointer suddenly loses its ability to go past the edge of the external display. It's as if the external monitor is my only display.

I'm unaware of any key combo I might be accidentally pressing to change the display arrangement. It seems to me that this is happening independent of anything I am doing, but am I making this happen myself, and if so, how?