Windows 8.1, also Linux Mint 17.3
Short description of error
Enabling 'Region Overlap' in the system preferences will allow the horizontal scrollbar in the VSE to scroll behind the properties panel, thus preventing it from being clicked on.
Exact steps for others to reproduce the error
Enable 'Region Overlap' in System preferences, also Triple Buffering if needed
Switch to a screen layout that includes the VSE in Sequencer mode
Enable the properties panel on the VSE
Scroll the scrollbar to the right, it will slide behind the properties panel and be inaccessible