Answer by Kamil Maciorowski for tmux - How to remove delay after pressing prefix key and Up key

It's basically the opposite of this question: How do I resize tmux pane by holding down prefix and arrow key for a while? Please read my answer there, it will hopefully help you understand what happens when you trigger the unwanted behavior.

Now it should be clear there are at least two solutions:

1. Adjust repeat-time. This will still allow you to keep switching panes while sending ↑ fast enough, but you can choose the maximum delay that is considered fast enough. Note the setting applies to all keys that may repeat.

2. Or change the binding that results in select-pane -U, so there is no -r in it. This is a line for ~/.tmux.conf:

   bind-key -T prefix Up select-pane -U

   And this will* turn the feature off as you requested.

^{* Remember that the file is parsed when a tmux server starts or when you explicitly request it (e.g. with tmux source-file ~/.tmux.conf in a shell inside tmux). You can always change the binding in the currently running tmux server by simply executing tmux bind-key … in a shell inside tmux.}