A mirrored sheet is a special class of sheet that is attached directly to a window on a display server. Grafts, for example, are always mirrored sheets. However, any sheet anywhere in a sheet hierarchy may be a mirrored sheet. A mirrored sheet will usually contain a reference to a window system object, called a mirror. For example, a mirrored sheet attached to a Motif server might have an X window system object stored in one of its slots. Allowing mirrored sheets at any point in the hierarchy enables the adaptive toolkit facilities.
Since not all sheets in the hierarchy have mirrors, there is no direct correspondence between the sheet hierarchy and the mirror hierarchy. However, on those display servers that support hierarchical windows, the hierarchies must be parallel. If a mirrored sheet is an ancestor of another mirrored sheet, their corresponding mirrors must have a similar ancestor/descendant relationship.
CLIM interacts with mirrors when it must display output or process events. On output, the mirrored sheet closest in ancestry to the sheet on which we wish to draw provides the mirror on which to draw. The mirror's drawing clipping region is set up to be the intersection of the user's clipping region and the sheet's region (both transformed to the appropriate coordinate system) for the duration of the output. On input, events are delivered from mirrors to the sheet hierarchy. The CLIM port must determine which sheet shall receive events based on information such as the location of the pointer.
In both of these cases, we must have a coordinate transformation that converts coordinates in the mirror (so-called "native" coordinates) into coordinates in the sheet and vice-versa.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017