renderer: queue updates as not high priority

This seems like a more sensible choice given that on macOS an update
to a slider while sliding can interfere with the slider's state.

Closes #63.

Author: Bogdanp

examples/issue-63.rkt

@@ -0,0 +1,33 @@
+#lang racket/base
+
+(require racket/gui/easy
+         racket/gui/easy/debugger
+         racket/gui/easy/operator)
+
+(define @a (@ 0))
+(define @b (@ 0))
+(define @b2 (obs-throttle @b))
+
+(module+ main
+  (start-debugger)
+  (render
+   (window
+    (hpanel
+     (text "Slider on @a")
+     (slider @a #:min-value 0 #:max-value 100
+             (λ (v) (:= @a v)))
+     (text (obs-map @a number->string)))
+
+    (hpanel
+     (text "Slider on @b")
+     (slider @b #:min-value 0 #:max-value 100
+             (λ (v) (:= @b v)))
+     (text (obs-map @b number->string))
+     (text (obs-map @b2 number->string)))
+
+    (hpanel
+     (text "Slider on throttled @b")
+     (slider @b2 #:min-value 0 #:max-value 100
+             (λ (v) (:= @b v)))
+     (text (obs-map @b number->string))
+     (text (obs-map @b2 number->string))))))

gui-easy-lib/gui/easy/private/observable.rkt

@@ -155,7 +155,9 @@
     (for ([obs (in-list (reverse (unbox (obs-observers-box o))))])
       (with-handlers ([exn:fail?
                        (lambda (e)
-                         ((error-display-handler) (exn-message e) e))])
+                         ((error-display-handler)
+                          (format "do-obs-update!: ~a" (exn-message e))
+                          e))])
         (obs v)))))
 
 (define (obs-update! o f)

gui-easy-lib/gui/easy/private/renderer.rkt

@@ -66,7 +66,8 @@
            (for-each do-remove-dependencies depss)
            (parameterize ([current-renderer this])
              (send tree destroy root))
-           (set! root #f)))))
+           (set! root #f)))
+       #;high-priority? #f))
 
     (define (do-add-dependencies deps tree root) ;; noqa
       (define s
@@ -75,7 +76,8 @@
                                  (gui:queue-callback
                                   (lambda ()
                                     (parameterize ([current-renderer this])
-                                      (send tree update root dep v)))))
+                                      (send tree update root dep v)))
+                                  #;high-priority? #f))
                                (begin0 proc
                                  (obs-observe! dep proc)))))
       (begin0 s

gui-easy-lib/info.rkt

@@ -1,7 +1,7 @@
 #lang info
 
 (define license 'BSD-3-Clause)
-(define version "0.20")
+(define version "0.20.1")
 (define collection "racket")
 (define deps
   '("base"