"Select to the beginning of the line" is wrong in DrRacket

[sorawee]

Consider:

(define (f x)
  (+ x x)|
  (+ x x))

where | is the caret. Press shift+home. This will select (+ x x)

(define (f x)
  [(+ x x)]
  (+ x x))

then delete it via backspace or cut (ctrl-x) but not the delete key. Now, we are in this state:

(define (f x)
  |
  (+ x x))

Everything so far is as expected. However, if we shift+home again, DrRacket selects incorrectly:

(define (f x)
[  
  (+ x] x))

Note however that if we move around (e.g., press left key and then right key) first before shift+home, the selection will be correct.

(define (f x)
[  ]
  (+ x x))

Also note that the length of the deleted (+ x x) which is 7 characters matches with the length of the over-selection \n (+ x

[lexi-lambda]

I’ve bumped into this bug—or at least a related bug—many times, but I’ve never taken the time to figure out exactly what steps I took to report it. Specifically, this part

Note however that if we move around (e.g., press left key and then right key) first before shift+home, the selection will be correct.

rings true for me. Something about certain sequences of move and delete actions seem to make DrRacket lose track of some piece of information, causing future actions to behave strangely, but just pressing left then right always seems to fix that strange desync.

[rfindler]

This appears to be a bug in extend-position. Here's two programs that should behave the same, but don't. (I'll keep poking around here, but this seemed like a useful piece to share.)

The property is that if you do (begin (send t set-position x) (send t extend-position y)) that should be the same as (send t set-position (min x y) (max x y)) and the other code is just revealing the difference between the two. At least, I think that's a valid property.

#lang racket/gui

(define lines
  (list
   "(define (f x)\n"
   "  (+ x x)\n"
   "  (+ x x))\n"))

(define (show t label)
  (define f (new frame% [label label] [width 400] [height 200]))
  (new editor-canvas% [parent f] [editor t])
  (send f show #t))

(let ()
  (define t (new text%))

  (send t insert (apply string-append lines))
  (send t set-position
        (+ (string-length (list-ref lines 0))
           (string-length (list-ref lines 1))
           -1 ;; before the newline
           ))
  (send t extend-position
        (+ (string-length (list-ref lines 0))
           2 ;; after the leading space
           ))
  (send t cut)
  (send t extend-position (string-length (list-ref lines 0)))
  (show t "1")
  (values (send t get-start-position)
          (send t get-end-position)))

(let ()
  (define t (new text%))

  (send t insert (apply string-append lines))
  (send t set-position
        (+ (string-length (list-ref lines 0))
           2 ;; after the leading space
           )
        (+ (string-length (list-ref lines 0))
           (string-length (list-ref lines 1))
           -1 ;; before the newline
           ))
  (send t cut)
  (send t extend-position (string-length (list-ref lines 0)))
  (show t "2")
  (values (send t get-start-position)
          (send t get-end-position)))