diff --git a/vncviewer/CConn.cxx b/vncviewer/CConn.cxx
index fb136b5..262dd2c 100644
--- a/vncviewer/CConn.cxx
+++ b/vncviewer/CConn.cxx
@@ -46,6 +46,7 @@
 #include "CConn.h"
 #include "OptionsDialog.h"
 #include "DesktopWindow.h"
+#include "PlatformPixelBuffer.h"
 #include "i18n.h"
 #include "parameters.h"
 #include "vncviewer.h"
@@ -336,6 +337,9 @@
 // one.
 void CConn::framebufferUpdateStart()
 {
+  ModifiablePixelBuffer* pb;
+  PlatformPixelBuffer* ppb;
+
   CConnection::framebufferUpdateStart();
 
   // Note: This might not be true if sync fences are supported
@@ -343,6 +347,22 @@
 
   requestNewUpdate();
 
+  // We might still be rendering the previous update
+  pb = getFramebuffer();
+  assert(pb != NULL);
+  ppb = dynamic_cast<PlatformPixelBuffer*>(pb);
+  assert(ppb != NULL);
+  if (ppb->isRendering()) {
+    // Need to stop monitoring the socket or we'll just busy loop
+    assert(sock != NULL);
+    Fl::remove_fd(sock->getFd());
+
+    while (ppb->isRendering())
+      run_mainloop();
+
+    Fl::add_fd(sock->getFd(), FL_READ | FL_EXCEPT, socketEvent, this);
+  }
+
   // Update the screen prematurely for very slow updates
   Fl::add_timeout(1.0, handleUpdateTimeout, this);
 }
diff --git a/vncviewer/PlatformPixelBuffer.cxx b/vncviewer/PlatformPixelBuffer.cxx
index 522bad3..876ab94 100644
--- a/vncviewer/PlatformPixelBuffer.cxx
+++ b/vncviewer/PlatformPixelBuffer.cxx
@@ -44,3 +44,8 @@
 
   return r;
 }
+
+bool PlatformPixelBuffer::isRendering(void)
+{
+  return false;
+}
diff --git a/vncviewer/PlatformPixelBuffer.h b/vncviewer/PlatformPixelBuffer.h
index 795273a..9f0e3b1 100644
--- a/vncviewer/PlatformPixelBuffer.h
+++ b/vncviewer/PlatformPixelBuffer.h
@@ -34,6 +34,8 @@
   virtual void draw(int src_x, int src_y, int x, int y, int w, int h) = 0;
   rfb::Rect getDamage(void);
 
+  virtual bool isRendering(void);
+
 protected:
   os::Mutex mutex;
   rfb::Region damage;
diff --git a/vncviewer/X11PixelBuffer.cxx b/vncviewer/X11PixelBuffer.cxx
index 9196fdc..046676e 100644
--- a/vncviewer/X11PixelBuffer.cxx
+++ b/vncviewer/X11PixelBuffer.cxx
@@ -21,8 +21,10 @@
 #include <config.h>
 #endif
 
+#include <assert.h>
 #include <stdlib.h>
 
+#include <FL/Fl.H>
 #include <FL/x.H>
 
 #include <rfb/LogWriter.h>
@@ -35,6 +37,8 @@
 
 static rfb::LogWriter vlog("X11PixelBuffer");
 
+std::list<X11PixelBuffer*> X11PixelBuffer::shmList;
+
 static PixelFormat display_pf()
 {
   int i;
@@ -97,7 +101,7 @@
 
 X11PixelBuffer::X11PixelBuffer(int width, int height) :
   PlatformPixelBuffer(display_pf(), width, height, NULL, 0),
-  shminfo(NULL), xim(NULL)
+  shminfo(NULL), xim(NULL), pendingPutImage(0)
 {
   // Might not be open at this point
   fl_open_display();
@@ -122,6 +126,8 @@
 {
   if (shminfo) {
     vlog.debug("Freeing shared memory XImage");
+    shmList.remove(this);
+    Fl::remove_system_handler(handleSystemEvent);
     shmdt(shminfo->shmaddr);
     shmctl(shminfo->shmid, IPC_RMID, 0);
     delete shminfo;
@@ -137,12 +143,18 @@
 
 void X11PixelBuffer::draw(int src_x, int src_y, int x, int y, int w, int h)
 {
-  if (shminfo)
-    XShmPutImage(fl_display, fl_window, fl_gc, xim, src_x, src_y, x, y, w, h, False);
-  else
+  if (shminfo) {
+    XShmPutImage(fl_display, fl_window, fl_gc, xim, src_x, src_y, x, y, w, h, True);
+    pendingPutImage++;
+  } else {
     XPutImage(fl_display, fl_window, fl_gc, xim, src_x, src_y, x, y, w, h);
+  }
 }
 
+bool X11PixelBuffer::isRendering(void)
+{
+  return pendingPutImage > 0;
+}
 
 static bool caughtError;
 
@@ -202,6 +214,11 @@
   if (caughtError)
     goto free_shmaddr;
 
+  // FLTK is a bit stupid and unreliable if you register the same
+  // callback with different data values.
+  Fl::add_system_handler(handleSystemEvent, NULL);
+  shmList.push_back(this);
+
   vlog.debug("Using shared memory XImage");
 
   return 1;
@@ -222,3 +239,31 @@
 
   return 0;
 }
+
+int X11PixelBuffer::handleSystemEvent(void* event, void* data)
+{
+  XEvent* xevent;
+  XShmCompletionEvent* shmevent;
+
+  std::list<X11PixelBuffer*>::iterator iter;
+
+  xevent = (XEvent*)event;
+  assert(xevent);
+
+  if (xevent->type != XShmGetEventBase(fl_display))
+    return 0;
+
+  shmevent = (XShmCompletionEvent*)event;
+
+  for (iter = shmList.begin();iter != shmList.end();++iter) {
+    if (shmevent->shmseg != (*iter)->shminfo->shmseg)
+      continue;
+
+    (*iter)->pendingPutImage--;
+    assert((*iter)->pendingPutImage >= 0);
+
+    return 1;
+  }
+
+  return 0;
+}
diff --git a/vncviewer/X11PixelBuffer.h b/vncviewer/X11PixelBuffer.h
index c2ffdc2..115984d 100644
--- a/vncviewer/X11PixelBuffer.h
+++ b/vncviewer/X11PixelBuffer.h
@@ -24,6 +24,8 @@
 #include <sys/shm.h>
 #include <X11/extensions/XShm.h>
 
+#include <list>
+
 #include "PlatformPixelBuffer.h"
 
 class X11PixelBuffer: public PlatformPixelBuffer {
@@ -33,12 +35,19 @@
 
   virtual void draw(int src_x, int src_y, int x, int y, int w, int h);
 
+  virtual bool isRendering(void);
+
 protected:
   int setupShm();
 
+  static int handleSystemEvent(void* event, void* data);
+
 protected:
   XShmSegmentInfo *shminfo;
   XImage *xim;
+  int pendingPutImage;
+
+  static std::list<X11PixelBuffer*> shmList;
 };