change backspace in overwrite mode to be more what one

might expect it to be

gui-lib/mred/private/wxme/text.rkt

@@ -612,7 +612,16 @@
                        (insert ch startpos (add1 startpos))
                        (insert ch)))])
         (case code
-          [(#\backspace) (delete)]
+          [(#\backspace)
+           (cond
+             [(and overwrite-mode?
+                   (= endpos startpos)
+                   (not (zero? startpos)))
+              (begin-edit-sequence)
+              (insert #\space (- startpos 1) startpos)
+              (set-position (- startpos 1) (- startpos 1))
+              (end-edit-sequence)]
+             [else (delete)])]
           [(#\rubout)
            (if (= endpos startpos)
                (when (endpos . < . len)

gui-test/tests/gracket/wxme.rkt

@@ -1561,6 +1561,31 @@
   (send t1 insert "Hello\tWorld")
   (send t1 get-extent (box 0) (box 0)))
 
+;; ----------------------------------------
+;; Overwrite mode
+(let ([t (new text%)])
+  (send t set-admin (new test-editor-admin%))
+  (send t insert "abcdef")
+  (send t set-position 3 3)
+  (define (type c) (send t on-default-char (new key-event% [key-code c])))
+  (send t set-overwrite-mode #t)
+  (type #\z)
+  (expect (send t get-start-position) 4)
+  (expect (send t get-text) "abczef")
+  (type #\backspace)
+  (expect (send t get-start-position) 3)
+  (expect (send t get-text) "abc ef")
+  
+  (send t set-position 1)
+  (type #\backspace)
+  (expect (send t get-start-position) 0)
+  (expect (send t get-text) " bc ef")
+  
+  (type #\backspace)
+  (expect (send t get-start-position) 0)
+  (expect (send t get-text) " bc ef"))
+
+
 ;; ----------------------------------------
 ;; Identity and contracts