How do I stop the "Properties" from disappearing?

[tmighty2]

I experience something strange since a few days:

The properties window is gone, also from the context menu.
Usually, when I right-click a control, it offers "Properties".
Clicking this item would open the properties window on the right, offering "Name", "Caption", "Index", etc.

For me, it's gone, and I don't know how to get it back.