<bmx-splitter>
Two panes and a divider the reader can move.
Drag the divider, or focus it with Tab and use the arrow keys. Double-click it to fold the list away and again to bring it back at the size it had.
Messages
A list narrow enough to be useful and wide enough to read. It will not go below 140 pixels, and past 60 more it folds away entirely.
Quarterly review
The pane that takes what is left. Drag towards the middle and the divider is pulled onto the halfway mark — the position a reader is usually aiming at, and a surprisingly hard one to hit by hand.
The list has 34% of the room.
Every property, event and method: https://docs.binarymission.co.uk/webcomponents/bmx/bmx-splitter.html