Turning it off
Mac App Store version
defaults write ~/Library/Group\ Containers/8482Q6EPL6.com.codeux.irc.textual/Library/Preferences/8482Q6EPL6.com.codeux.irc.textual "Tab Completion -> Do Not Use Whitespace for Missing Completion Suffix" -bool YES
Non-Mac App Store version
defaults write com.codeux.apps.textual "Tab Completion -> Do Not Use Whitespace for Missing Completion Suffix" -bool YES
Turning it back on
Mac App Store version
defaults delete ~/Library/Group\ Containers/8482Q6EPL6.com.codeux.irc.textual/Library/Preferences/8482Q6EPL6.com.codeux.irc.textual "Tab Completion -> Do Not Use Whitespace for Missing Completion Suffix"
Non-Mac App Store version
defaults delete com.codeux.apps.textual "Tab Completion -> Do Not Use Whitespace for Missing Completion Suffix"