Enter copy mode and scroll one page up
copy mode
// what it does
Prefix then PageUp is bound by default to `copy-mode -u`, which enters copy mode and immediately scrolls up one full page. It's a one-keystroke shortcut for the common case of "I need to see what just scrolled off the top," saving the separate prefix-[ then PageUp steps.
// shortcut
Ctrl + bPgUp
// gotcha
You press and release the prefix, then press PageUp — it is not a held chord. After it fires you're already in copy mode, so further PageUp presses (without prefix) keep scrolling.