Adding Fl_Input comments.

src/Fl_Input.cxx

@@ -320,13 +320,30 @@ int Fl_Input::kf_copy_cut() {
   class. It handles compose key sequences and can also be used e.g. in
   Fl_Multiline_Input, Fl_Float_Input and several more derived classes.
 
-  The details are way too complicated to be documented here and can be
-  changed as required. If in doubt, please consult the source code.
+  The method first checks in Fl::compose if the keystroke is a text entry or
+  a control key. If it is text, the method inserts the composed characters into
+  the input field, taking into account the input type (e.g., numeric fields).
+
+  If the keystroke is a control key as determined by Fl::compose, the method
+  handles key combinations for Insert, Enter, and Tab depending on the
+  widget's input_type().
+
+  The method then checks for Ctrl key combinations, such as Ctrl-A, Ctrl-C, 
+  Ctrl-V, Ctrl-X, and Ctrl-Z, which are commonly used for select all, copy,
+  paste, cut, and undo operations.
+
+  Finally, the method checks for ASCII control characters, such as Ctrl-H,
+  Ctrl-I, Ctrl-J, Ctrl-L, and Ctrl-M, which can be used to insert literal
+  control characters into the input field.
+
+  If none of the above cases match, the method returns 0, indicating that the
+  keystroke was not handled.
 
   \returns  1 if the keystroke is handled by us, 0 if not.
 */
 int Fl_Input::handle_key() {
 
+  // This is unicode safe: only character codes < 128 are queried
   char ascii = Fl::event_text()[0];
 
   int del;