diff --git a/chaos/gui.rkt b/chaos/gui.rkt
index f3f8f33..53128ac 100644
--- a/chaos/gui.rkt
+++ b/chaos/gui.rkt
@@ -6,17 +6,30 @@
          data/queue
          lux/chaos)
 
-(struct gui (depth-box events-box fps drawer frame refresh!)
+(struct *sbox (sema box))
+(define (sbox v)
+  (*sbox (make-semaphore 1) (box v)))
+(define (sbox-swap! sb new)
+  (match-define (*sbox sema b) sb)
+  (call-with-semaphore sema
+    (λ ()
+      (begin0 (unbox b)
+        (set-box! b new)))))
+(define (sbox-poke sb f)
+  (match-define (*sbox sema b) sb)
+  (call-with-semaphore sema
+    (λ () (f (unbox b)))))
+
+(struct gui (depth-box events-sbox fps drawer frame refresh!)
         #:methods gen:chaos
         [(define (chaos-fps c)
            (gui-fps c))
          (define (chaos-yield c e)
            (yield e))
          (define (chaos-inputs c)
-           (define eb (gui-events-box c))
+           (define eb (gui-events-sbox c))
            (define new-q (make-queue))
-           (define q (unbox eb))
-           (set-box! eb new-q)
+           (define q (sbox-swap! eb new-q))
            (in-queue q))
          (define (chaos-output! c o)
            (set-box! (gui-drawer c) o)
@@ -36,17 +49,17 @@
                   #:mode [mode 'draw]
                   #:width [init-w 800]
                   #:height [init-h 600])
-  (define events-box (box (make-queue)))
+  (define events-box (sbox (make-queue)))
   (define gframe%
     (class frame%
       (define/override (on-size w h)
         (refresh!))
       (define/augment (on-close)
-        (enqueue! (unbox events-box) 'close))
+        (sbox-poke events-box (λ (q) (enqueue! q 'close))))
       (define/override (on-subwindow-char w ke)
-        (enqueue! (unbox events-box) ke))
+        (sbox-poke events-box (λ (q) (enqueue! q ke))))
       (define/override (on-subwindow-event w me)
-        (enqueue! (unbox events-box) me))
+        (sbox-poke events-box (λ (q) (enqueue! q me))))
       (super-new)))
 
   (define drawer (box void))
diff --git a/chaos/gui/val.rkt b/chaos/gui/val.rkt
index 15f2484..3e75bc3 100644
--- a/chaos/gui/val.rkt
+++ b/chaos/gui/val.rkt
@@ -6,7 +6,7 @@
          pict/convert
          lux/chaos)
 
-(struct gui/val (scale? g)
+(struct gui/val (scale? g [last #:mutable])
         #:methods gen:chaos
         [(define/generic super-fps chaos-fps)
          (define/generic super-yield chaos-yield)
@@ -21,31 +21,33 @@
          (define (chaos-inputs c)
            (super-inputs (gui/val-g c)))
          (define (chaos-output! c o)
-           (define p (pict-convert o))
-           (super-output!
-            (gui/val-g c)
-            (λ (w h dc)
-              (parameterize ([dc-for-text-size dc])
-                (send dc set-background "black")
-                (send dc clear)
-                (define sp
-                  (if (gui/val-scale? c)
-                      (scale-to-fit p w h)
-                      p))
-                (define spw (pict-width sp))
-                (define left (/ (- w spw) 2))
-                (define sph (pict-height sp))
-                (define top (/ (- h sph) 2))
-                (send dc set-brush "white" 'solid)
-                (send dc draw-rectangle left top spw sph)
-                (draw-pict sp dc left top)))))
+           (unless (eq? o (gui/val-last c))
+             (set-gui/val-last! c o)
+             (define p (pict-convert o))
+             (super-output!
+              (gui/val-g c)
+              (λ (w h dc)
+                (parameterize ([dc-for-text-size dc])
+                  (send dc set-background "black")
+                  (send dc clear)
+                  (define sp
+                    (if (gui/val-scale? c)
+                        (scale-to-fit p w h)
+                        p))
+                  (define spw (pict-width sp))
+                  (define left (/ (- w spw) 2))
+                  (define sph (pict-height sp))
+                  (define top (/ (- h sph) 2))
+                  (send dc set-brush "white" 'solid)
+                  (send dc draw-rectangle left top spw sph)
+                  (draw-pict sp dc left top))))))
          (define (chaos-label! c l)
            (super-label! (gui/val-g c) l))
          (define (chaos-swap! c t)
            (super-swap! (gui/val-g c) t))])
 
 (define (make-gui/value g #:scale? [scale? #t])
-  (gui/val scale? g))
+  (gui/val scale? g #f))
 (provide
  (contract-out
   [make-gui/value