Debugging a focus issue

[pkrumins]

Hi Greg,

I have recently been improving my terminal workflow and noticed an issue with urxvt and this plugin. Sometimes, when vim is running in plain urxvt (without tmux), the focus events are not sent to vim. I'm not familiar with how the terminal focus events work and wanted to ask if you could share any insight about how to debug this problem. I'll then try to fix it for urxvt and submit a PR. Thanks!

Peter