About this Add-on
Double click on the thread pane splitter causes it to automatically slide in to optimal position.
Right click on the thread pane splitter toggles the autosplit feature (dark color means no autosplit).
The maximal size of the thread pane can be set in the addon preferences.
For new versions also see the beta channel: https://addons.mozilla.org/thunderbird/addon/autoslide/#beta-channel