Try this. In RoboHelp HTML, click to expand your Windows
folder. You do this in the Project Manager on the left side. See
After expanding it, you should see at least one window there.
Double-click this window to see its properties. Once you do this,
you should find an area just under the large preview section that
is labeled "Tri-pane Tabs and Windows". In this area are check
boxes that should assist you in achieving your goal.