improve position adjustment after hitting tab

Specifically, after indentation, if move the insertion point to the
first nonwhitespace on the line (in the case that there is an
insertion point instead of a selection), when the insertion point was
already in the whitespace at the start of the line

Author: rfindler

drracket-core-lib/drracket/private/module-language-tools.rkt

@@ -216,6 +216,7 @@
     (mixin (text:basic<%>) (ml-tools-text<%>)
       (inherit position-paragraph paragraph-start-position
                delete insert
+               set-position paragraph-end-position get-character
                begin-edit-sequence end-edit-sequence)
       (define/public (range-indent range-indentation-function start end)
         (define substs (range-indentation-function this start end))
@@ -227,13 +228,38 @@
                (let loop ([substs substs] [line start-line])
                  (unless (or (null? substs)
                              (line . > . end-line))
-                   (define pos (paragraph-start-position line))
+                   (define para-start-pos (paragraph-start-position line))
                    (define del-amt (caar substs))
                    (define insert-str (cadar substs))
+                   (define in-whitespace-before-edit?
+                     (cond
+                       [(= start end)
+                        (define para-end (paragraph-end-position start-line))
+                        (let loop ([pos para-start-pos])
+                          (cond
+                            [(< pos para-end)
+                             (define c (get-character pos))
+                             (cond
+                               [(char-whitespace? c) (loop (+ pos 1))]
+                               [else
+                                (<= para-start-pos start pos)])]
+                            [else #f]))]
+                       [else #f]))
                    (unless (zero? del-amt)
-                     (delete pos (+ pos del-amt)))
+                     (delete para-start-pos (+ para-start-pos del-amt)))
                    (unless (equal? insert-str "")
-                     (insert insert-str pos))
+                     (insert insert-str para-start-pos))
+                   (when (and (= start end)
+                              in-whitespace-before-edit?)
+                     (define para-end (paragraph-end-position start-line))
+                     (let loop ([pos para-start-pos])
+                       (cond
+                         [(< pos para-end)
+                          (define c (get-character pos))
+                          (cond
+                            [(char-whitespace? c) (loop (+ pos 1))]
+                            [else (set-position pos pos)])]
+                         [else (void)])))
                    (loop (cdr substs) (add1 line))))
                (end-edit-sequence)
                #t)))