2.3 The viewport hierarchy

2.3.2 Rules for viewport or window occlusion

The basic rules for viewport or window occlusion in the viewport hierarchy are as follows:

A viewport occludes its ancestors.
A viewport occludes the viewports that its ancestors occlude and the descendants of those viewports.
A viewport is occluded by the viewports that occlude its ancestors and by the descendants of those viewports.
If two viewports are siblings, the viewport that is closest to the top of the sibling stack and all of its descendants occlude the viewport that is further down in the stack and all of its descendants.
A viewport is clipped to fit within the boundaries of its parent.

The Window Tool Kit - 9 SEP 1996