I'm on Slackware 14.2, running the standard tmux package (tmux-2.1-x86_64-3) and the w3m Slackbuild (w3m-git20170104-x86_64-1_SBo).

Recently, I've started running into the following issue: starting w3m in either a screen or a tmux pane will work properly for a bit, and then keys will cease to register (no key presses have any effect on w3m). Killing the w3m process reveals that the keys are being sent to the right tmux pane, but they do not register in w3m even when the latter was running in that pane.

Closing the w3m instance and starting a new one in a different tmux session / window / pane does not help: only restarts restore functionality (temporarily).

EDIT:

I've found a way to make it work without rebooting, but I would still very much appreciate any insight into what the problem might be. What works:

Switch to a different TTY (Ctrl+Alt+F1 to F6), attach to the tmux session there, and start w3m in one of the panes. Then, detaching and moving back to the original session restores functionality next time w3m is started in one of the panes.