Hi...
If I understand correctly the question, expandable are are managed by setting them to 100%...
For that, put a cell at the desired place, and when it's selected, change the size information in the right column area...
To study that more closely you can just create an empty page using some of the webdev existing templates... Some oh them have exapndable area aither vertically or horizontally
Best regards
Fabrice Harari
WinDev, WebDev, WinDev Mobile consulting