Stopping Textual putting a space after tab completion
When your tab-completion field in Textual is empty, it automatically inserts a space after the nickname when you tab complete - this behaviour was added in 5.2.4.
If you’re using a version of Textual newer than or equal to 5.2.5, there’s a hidden preference that lets you turn this off.
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"