What is causing the mouse scroll wheel to input characters into my code?

The short reason: the mouse wheel itself doesn’t normally write characters to stdin, but many terminal emulators can (and do) emit escape sequences for mouse events when mouse reporting is enabled. Because your KeyPoller puts the terminal into raw/unbuffered mode and simply reads stdin, those escape sequences (from mouse wheel or mouse clicks) appear as single characters or short sequences to your reader.

Common mouse-event sequences start with ESC (\x1b) — examples include old \x1b[M... (X10) or newer SGR/URXVT sequences \x1b[