racket/draw: add #:eventspace' argument to open-output-text-editor'

The argument is `(curent-eventspace)' by default, which makes ports
work better with threads.

Closes PR 12749

original commit: b30374824a7d79b96443da6e0e4223dd72da597b

collects/mred/private/snipfile.rkt

@@ -10,7 +10,7 @@
   (provide open-input-text-editor
            open-input-graphical-file
            text-editor-load-handler
-           open-output-text-editor )
+           open-output-text-editor)
 
   ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
@@ -312,7 +312,8 @@
       p))
 
   (define open-output-text-editor 
-    (lambda (text [start 'end] [special-filter values] [port-name text])
+    (lambda (text [start 'end] [special-filter values] [port-name text]
+                  #:eventspace [eventspace (wx:current-eventspace)])
       (define pos (if (eq? start 'end)
 		      (send text last-position)
 		      (min start
@@ -322,9 +323,16 @@
       (define raw-buffer (make-bytes 128))
       (define utf8-buffer (make-bytes 128))
       (define (show s)
+        (define (insert)
           (send text begin-edit-sequence)
           (send text insert s pos)
-        (send text end-edit-sequence)
+          (send text end-edit-sequence))
+        (if (and eventspace
+                 (and (not (eq? (current-thread) 
+                                (wx:eventspace-handler-thread eventspace)))))
+            (parameterize ([wx:current-eventspace eventspace])
+              (wx:queue-callback insert #f))
+            (insert))
 	(set! pos (+ (string-length s) pos)))
       (define (flush-text)
 	(let ([cnt (peek-bytes-avail!* raw-buffer 0 #f in)])

collects/scribblings/gui/editor-funcs.scrbl

@@ -266,7 +266,8 @@ if using those methods are always safe.)
 @defproc[(open-output-text-editor [text-editor (is-a?/c text%)]
                                   [start-position (or/c exact-nonnegative-integer? (one/of 'end)) 'end]
                                   [special-filter (any/c . -> . any/c) (lambda (x) x)]
-                                  [port-name any/c text-editor])
+                                  [port-name any/c text-editor]
+                                  [#:eventspace eventspace (or/c eventspace? #f) (current-eventspace)])
          output-port]{
 
 Creates an output port that delivers its content to @racket[text-editor].
@@ -284,9 +285,20 @@ If line counting is enabled for the resulting output port, then the
  port will report the line, offset from the line's start, and position
  within the editor at which the port writes data.
 
+If @racket[eventspace] is not @racket[#f], then when the output port
+ is used in a thread other than @racket[eventspace]'s handler thread,
+ content is delivered to @racket[text-editor] through a low-priority
+ callback in @racket[eventspace].  Thus, if @racket[eventspace]
+ corresponds to the eventspace for the editor's @tech{displays},
+ writing to the output port is safe from any thread.
 
+If @racket[eventspace] is @racket[#f], beware that the port is only
+ weakly thread-safe. Content is delivered to @racket[text-editor] in
+ an @tech{edit sequence}, but an edit sequence is not enough
+ synchronization if, for example, the editor is displayed in an
+ enabled @racket[editor-canvas%]. See @secref["editorthreads"] for
+ more information.}
 
-}
 
 @defproc[(read-editor-global-footer [in (is-a?/c editor-stream-in%)])
          boolean?]{

collects/scribblings/gui/editor-overview.scrbl

@@ -735,7 +735,7 @@ An editor supports certain concurrent patterns
 @itemize[
 
  @item{When an editor's @method[editor<%> refresh] method is
- called during an edit sequence (which is started by
+ called during an @deftech{edit sequence} (which is started by
  @method[editor<%> begin-edit-sequence] and ended with
  @method[editor<%> end-edit-sequence]), the requested refresh
  region is recorded, but the refresh is not performed. Instead, the