removed flashing scroll bar when expanding content

[ondrejkonec]

Opening this pull request because of legacy code in CSS and also the reaction to #1224

I already tried to fix this issue when it was reported here #1142.

expandable.less code was changed a lot (during adding local component custom properties and cleanup code initiative) so this PR is fixing only scrollbar issue.

I think there is a lot of outdated code (used to fix browser bugs, but that is no longer current) so I think this "component" can be simplified even more, and I'm happy to get into it.