Constrain left pane container to height: 100% so that scrollbar show up when content wants to overflow.
The `main` classname seems too generic, but I left it alone (so I don't break anything), since this part will eventually be ported to React.
.main CSS class has a naming conflict with Moncao editor CSS classes and this is causing A11y issues with Moncao editor.
This class should no longer be used since we moved to the new tree component in REACT, so I am removing it. From my testing, this is not affecting anything.
If we find any styling issue later, we should fix without adding back this class.
