




The default composition callback of the editor. Composition here means composing input characters into other characters by an input method.
The function editor-pane-default-composition-callback is the default 
composition-callback
 of editor-pane. It may also be called by your program. 
When called with 
what
 = :start, editor-pane-default-composition-callback sets the composition placement in the editor by calling set-composition-placement, and also makes it move the composition window following the user's mouse cursor movement 
When called with 
what
 = :end, it stops the following of the mouse cursor. 
When called with a list (which needs to be a plist), editor-pane-default-composition-callback checks if it contains a keyword/value pair for :string-face-lists, and if it does displays it in the editor temporarily (until the next call to it). See the entry for  output-pane for the description of the value 
string-face-lists
.
By default, editor-pane-default-composition-callback uses the faces that are supplied in 
string-face-lists
, but if the plist contains :selection-needs-face and :selected-range, it displays the selected range with a different face, by merging *editor-pane-composition-selected-range-face-plist* into the given face of the selected range. 
This can be overridden by setting the 
composition-face
 in the editor-pane, or the global *editor-pane-default-composition-face* if the 
composition-face
 of the pane is :default.  If 
composition-face
 is a true value then the exact behavior depends on its type:
This is appended to each face plist in the the string-face-lists . In other words, it provides default values for the attributes of the face.
Overrides the supplied face completely.
For 
string-face-list
, funcalls it with two arguments, the pane and the supplied face plist, and uses the result (which may be an editor:face or a face plist). 
editor-pane-default-composition-callback is the default value of 
composition-callback
 for editor-pane. This can be overridden by passing :composition-callback or using output-pane-composition-callback (see entry for output-pane). 
The user-supplied callback may call editor-pane-default-composition-callback to do the actual display, potentially after modifying the argument when it is a plist.
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017