diff --git a/doomsday/client/client.pro b/doomsday/client/client.pro
index d8245b0d52..2c7dddbfcd 100644
--- a/doomsday/client/client.pro
+++ b/doomsday/client/client.pro
@@ -360,6 +360,7 @@ DENG_HEADERS += \
     include/ui/ui_main.h \
     include/ui/ui_panel.h \
     include/ui/window.h \
+    include/ui/windowsystem.h \
     include/ui/zonedebug.h \
     include/updater.h \
     include/uri.hh \
@@ -636,6 +637,7 @@ SOURCES += \
     src/ui/ui_main.cpp \
     src/ui/ui_panel.cpp \
     src/ui/window.cpp \
+    src/ui/windowsystem.cpp \
     src/ui/zonedebug.cpp \
     src/updater/downloaddialog.cpp \
     src/updater/processcheckdialog.cpp \
diff --git a/doomsday/client/include/dd_loop.h b/doomsday/client/include/dd_loop.h
index 6ad298dad0..7bdf8b6d62 100644
--- a/doomsday/client/include/dd_loop.h
+++ b/doomsday/client/include/dd_loop.h
@@ -47,6 +47,13 @@ int DD_GameLoop(void);
  */
 void DD_GameLoopCallback(void);
 
+/**
+ * Runs one or more tics depending on how much time has passed since the
+ * previous call to this function. This gets called once per each main loop
+ * iteration. Finishes as quickly as possible.
+ */
+void Loop_RunTics(void);
+
 /**
  * Window drawing callback.
  *
diff --git a/doomsday/client/include/ui/canvas.h b/doomsday/client/include/ui/canvas.h
index 2bf6a6b794..d092d9fe94 100644
--- a/doomsday/client/include/ui/canvas.h
+++ b/doomsday/client/include/ui/canvas.h
@@ -74,13 +74,6 @@ class Canvas : public QGLWidget
      */
     void setDrawFunc(void (*canvasDrawFunc)(Canvas&));
 
-    /**
-     * Sets the callback function that is called after the size of the canvas changes.
-     *
-     * @param canvasResizedFunc  Callback.
-     */
-    void setResizedFunc(void (*canvasResizedFunc)(Canvas&));
-
     /**
      * Sets the callback function that is called when the window's focus state changes.
      * The callback is given @c true or @c false as argument, with
diff --git a/doomsday/client/include/ui/window.h b/doomsday/client/include/ui/window.h
index 55322f03ff..21ecc0a32f 100644
--- a/doomsday/client/include/ui/window.h
+++ b/doomsday/client/include/ui/window.h
@@ -344,4 +344,15 @@ QWidget* Window_Widget(Window* wnd);
 
 CanvasWindow *Window_CanvasWindow(Window *wnd);
 
+/**
+ * Utility to call after changing the size of a CanvasWindow. This will update
+ * the Window state.
+ *
+ * @param win  Window instance.
+ *
+ * @deprecated In the future, size management will be done internally in
+ * CanvasWindow/WindowSystem.
+ */
+void Window_UpdateAfterResize(Window *win);
+
 #endif /* LIBDENG_SYS_WINDOW_H */
diff --git a/doomsday/client/include/ui/windowsystem.h b/doomsday/client/include/ui/windowsystem.h
new file mode 100644
index 0000000000..08df8ba4ef
--- /dev/null
+++ b/doomsday/client/include/ui/windowsystem.h
@@ -0,0 +1,47 @@
+/** @file windowsystem.h  Window management subsystem.
+ *
+ * @authors Copyright (c) 2013 Jaakko Keränen <jaakko.keranen@iki.fi>
+ *
+ * @par License
+ * GPL: http://www.gnu.org/licenses/gpl.html
+ *
+ * <small>This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version. This program is distributed in the hope that it
+ * will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details. You should have received a copy of the GNU
+ * General Public License along with this program; if not, see:
+ * http://www.gnu.org/licenses</small> 
+ */
+
+#ifndef CLIENT_WINDOWSYSTEM_H
+#define CLIENT_WINDOWSYSTEM_H
+
+#include <de/System>
+
+/**
+ * Window management subsystem.
+ *
+ * The window system processes events produced by the input drivers. In
+ * practice, the events are passed to the widgets in the windows.
+ *
+ * @ingroup gui
+ */
+class WindowSystem : public de::System
+{
+public:
+    WindowSystem();
+
+    ~WindowSystem();
+
+    bool processEvent(de::Event const &);
+
+    void timeChanged(de::Clock const &);
+
+private:
+    DENG2_PRIVATE(d)
+};
+
+#endif // CLIENT_WINDOWSYSTEM_H
diff --git a/doomsday/client/src/clientapp.cpp b/doomsday/client/src/clientapp.cpp
index 48ee07a8ef..e6c0753df6 100644
--- a/doomsday/client/src/clientapp.cpp
+++ b/doomsday/client/src/clientapp.cpp
@@ -34,6 +34,7 @@
 #include "con_main.h"
 #include "ui/displaymode.h"
 #include "sys_system.h"
+#include "ui/windowsystem.h"
 #include "ui/window.h"
 #include "updater.h"
 
@@ -66,6 +67,7 @@ DENG2_PIMPL(ClientApp)
 {
     LegacyCore *de2LegacyCore;
     QMenuBar *menuBar;
+    WindowSystem winSys;
     ServerLink *svLink;
 
     Instance(Public *i)
@@ -124,6 +126,9 @@ ClientApp::ClientApp(int &argc, char **argv)
     QCoreApplication::setApplicationVersion (DOOMSDAY_VERSION_BASE);
 
     setTerminateFunc(handleLegacyCoreTerminate);
+
+    // Subsystems.
+    addSystem(d->winSys);
 }
 
 ClientApp::~ClientApp()
diff --git a/doomsday/client/src/dd_loop.cpp b/doomsday/client/src/dd_loop.cpp
index 5d748f080e..3cec92bdf3 100644
--- a/doomsday/client/src/dd_loop.cpp
+++ b/doomsday/client/src/dd_loop.cpp
@@ -47,13 +47,6 @@
  */
 #define MAX_FRAME_TIME (1.0/MIN_TIC_RATE)
 
-/**
- * Maximum number of milliseconds spent uploading textures at the beginning
- * of a frame. Note that non-uploaded textures will appear as pure white
- * until their content gets uploaded (you should precache them).
- */
-#define FRAME_DEFERRED_UPLOAD_TIMEOUT 20
-
 int maxFrameRate = 120; // Zero means 'unlimited'.
 // Refresh frame count (independant of the viewport-specific frameCount).
 int rFrameCount = 0;
@@ -81,8 +74,6 @@ static int timeDeltasIndex = 0;
 
 static float realFrameTimePos = 0;
 
-static void runTics(void);
-
 void DD_RegisterLoop(void)
 {
     C_VAR_BYTE("input-sharp-lateprocessing", &processSharpEventsAfterTickers, 0, 0, 1);
@@ -130,7 +121,7 @@ void DD_GameLoopCallback(void)
 
         LegacyCore_SetLoopRate(count || !noninteractive? 35 : 3);
 
-        runTics();
+        Loop_RunTics();
 
         // Update clients at regular intervals.
         Sv_TransmitFrame();
@@ -139,27 +130,18 @@ void DD_GameLoopCallback(void)
 
 #ifdef __CLIENT__
     {
-        // Normal client-side/singleplayer mode.
-        //assert(!novideo);
-
-        // We may be performing GL operations.
-        Window_GLActivate(Window_Main());
-
-        // Run at least one (fractional) tic.
-        runTics();
-
-        // We may have received a Quit message from the windowing system
-        // during events/tics processing.
-        if(Sys_IsShuttingDown())
-            return;
-
-        GL_ProcessDeferredTasks(FRAME_DEFERRED_UPLOAD_TIMEOUT);
+        /**
+         * @todo The appropriate way to update the window is to have a
+         * repeating GuiApp::refresh() method posting window update events
+         * continually. We are drawing from here because LegacyCore is still
+         * controlling the main loop.
+         */
 
         // Request update of window contents.
         Window_Draw(Window_Main());
 
         // After the first frame, start timedemo.
-        DD_CheckTimeDemo();
+        //DD_CheckTimeDemo();
     }
 #endif
 }
@@ -516,12 +498,7 @@ timespan_t DD_LatestRunTicsStartTime(void)
     return lastRunTicsTime;
 }
 
-/**
- * Runs one or more tics depending on how much time has passed since the
- * previous call to this function. This gets called once per each main loop
- * iteration. Finishes as quickly as possible.
- */
-static void runTics(void)
+void Loop_RunTics(void)
 {
     double elapsedTime, ticLength, nowTime;
 
diff --git a/doomsday/client/src/gl/gl_defer.cpp b/doomsday/client/src/gl/gl_defer.cpp
index fc0dd89fd1..c0f98c1766 100644
--- a/doomsday/client/src/gl/gl_defer.cpp
+++ b/doomsday/client/src/gl/gl_defer.cpp
@@ -407,10 +407,7 @@ void GL_ProcessDeferredTasks(uint timeOutMilliSeconds)
     deferredtask_t* d;
     uint startTime;
 
-    if(novideo) return;
-
-    if(!inited)
-        Con_Error("GL_ProcessDeferredTasks: Deferred GL task system not initialized.");
+    if(novideo || !inited) return;
 
     LIBDENG_ASSERT_IN_MAIN_THREAD();
     LIBDENG_ASSERT_GL_CONTEXT_ACTIVE();
diff --git a/doomsday/client/src/network/net_buf.cpp b/doomsday/client/src/network/net_buf.cpp
index 7f6f698a0b..8cacf15cbf 100644
--- a/doomsday/client/src/network/net_buf.cpp
+++ b/doomsday/client/src/network/net_buf.cpp
@@ -210,6 +210,8 @@ void N_ReleaseMessage(netmessage_t *msg)
  */
 void N_ClearMessages(void)
 {
+    if(!msgMutex) return; // Not initialized yet.
+
     netmessage_t *msg;
     float oldSim = netSimulatedLatencySeconds;
 
diff --git a/doomsday/client/src/network/sys_network.cpp b/doomsday/client/src/network/sys_network.cpp
index fb92402a6a..d52473f2e8 100644
--- a/doomsday/client/src/network/sys_network.cpp
+++ b/doomsday/client/src/network/sys_network.cpp
@@ -1,4 +1,4 @@
-/** @file sys_network.cpp Low-level network socket routines.
+/** @file sys_network.cpp Low-level network socket routines (deprecated).
  * @ingroup network
  *
  * @todo Remove this source file entirely once dependent code is revised.
diff --git a/doomsday/client/src/ui/canvas.cpp b/doomsday/client/src/ui/canvas.cpp
index 06f4162711..ea39fd4a6a 100644
--- a/doomsday/client/src/ui/canvas.cpp
+++ b/doomsday/client/src/ui/canvas.cpp
@@ -69,7 +69,6 @@ struct Canvas::Instance
     QSize currentSize;
     void (*initCallback)(Canvas&);
     void (*drawCallback)(Canvas&);
-    void (*resizedCallback)(Canvas&);
     void (*focusCallback)(Canvas&, bool);
     bool cursorHidden;
     bool mouseGrabbed;
@@ -84,7 +83,6 @@ struct Canvas::Instance
         : self(c),
           initNotified(false), initCallback(0),
           drawCallback(0),
-          resizedCallback(0),
           focusCallback(0),
           cursorHidden(false),
           mouseGrabbed(false)
@@ -197,11 +195,6 @@ void Canvas::setDrawFunc(void (*canvasDrawFunc)(Canvas&))
     d->drawCallback = canvasDrawFunc;
 }
 
-void Canvas::setResizedFunc(void (*canvasResizedFunc)(Canvas&))
-{
-    d->resizedCallback = canvasResizedFunc;
-}
-
 void Canvas::setFocusFunc(void (*canvasFocusChanged)(Canvas&, bool))
 {
     d->focusCallback = canvasFocusChanged;
@@ -211,7 +204,6 @@ void Canvas::useCallbacksFrom(Canvas &other)
 {
     d->drawCallback = other.d->drawCallback;
     d->focusCallback = other.d->focusCallback;
-    d->resizedCallback = other.d->resizedCallback;
 }
 
 QImage Canvas::grabImage(const QSize& outputSize)
@@ -296,11 +288,6 @@ void Canvas::resizeGL(int w, int h)
     if(d->currentSize != newSize)
     {
         d->currentSize = newSize;
-
-        if(d->resizedCallback)
-        {
-            d->resizedCallback(*this);
-        }
     }
 }
 
diff --git a/doomsday/client/src/ui/canvaswindow.cpp b/doomsday/client/src/ui/canvaswindow.cpp
index ddc0706daa..949d637c60 100644
--- a/doomsday/client/src/ui/canvaswindow.cpp
+++ b/doomsday/client/src/ui/canvaswindow.cpp
@@ -219,13 +219,17 @@ void CanvasWindow::moveEvent(QMoveEvent *ev)
     }
 }
 
-void CanvasWindow::resizeEvent(QResizeEvent *)
+void CanvasWindow::resizeEvent(QResizeEvent *ev)
 {
+    QMainWindow::resizeEvent(ev);
+
     LOG_AS("CanvasWindow");
 
     de::Vector2i size(width(), height());
     LOG_DEBUG("Resized ") << size.asText();
 
+    Window_UpdateAfterResize(Window_Main()); /// @todo remove this
+
     d->rootWidget.setViewSize(size);
 }
 
diff --git a/doomsday/client/src/ui/legacywidget.cpp b/doomsday/client/src/ui/legacywidget.cpp
index f840460fa7..f4cc2cba36 100644
--- a/doomsday/client/src/ui/legacywidget.cpp
+++ b/doomsday/client/src/ui/legacywidget.cpp
@@ -22,12 +22,22 @@
 #include "ui/ui2_main.h"
 #include "ui/busyvisual.h"
 #include "dd_main.h"
+#include "dd_loop.h"
+#include "sys_system.h"
 #include "map/gamemap.h"
 #include "network/net_main.h"
 #include "render/rend_list.h"
 #include "render/rend_console.h"
 #include "audio/s_main.h"
 #include "gl/sys_opengl.h"
+#include "gl/gl_defer.h"
+
+/**
+ * Maximum number of milliseconds spent uploading textures at the beginning
+ * of a frame. Note that non-uploaded textures will appear as pure white
+ * until their content gets uploaded (you should precache them).
+ */
+#define FRAME_DEFERRED_UPLOAD_TIMEOUT 20
 
 boolean drawGame = true; // If false the game viewport won't be rendered
 
@@ -50,10 +60,44 @@ LegacyWidget::~LegacyWidget()
 
 void LegacyWidget::viewResized()
 {
+    LOG_AS("LegacyWidget");
+    LOG_DEBUG("View resized to ") << root().viewSize().asText();
+
+    // Update viewports.
+    R_SetViewGrid(0, 0);
+    if(BusyMode_Active() || UI_IsActive() || !App_GameLoaded())
+    {
+        // Update for busy mode.
+        R_UseViewPort(0);
+    }
+    R_LoadSystemFonts();
+    if(UI_IsActive())
+    {
+        UI_UpdatePageLayout();
+    }
 }
 
 void LegacyWidget::update()
-{
+{    
+    if(BusyMode_Active())
+    {
+        /// @todo During busy mode, a BusyWidget should be in the widget
+        /// tree instead of a LegacyWidget.
+        return;
+    }
+
+    // We may be performing GL operations.
+    Window_GLActivate(Window_Main());
+
+    // Run at least one (fractional) tic.
+    Loop_RunTics();
+
+    // We may have received a Quit message from the windowing system
+    // during events/tics processing.
+    if(Sys_IsShuttingDown())
+        return;
+
+    GL_ProcessDeferredTasks(FRAME_DEFERRED_UPLOAD_TIMEOUT);
 }
 
 void LegacyWidget::draw()
diff --git a/doomsday/client/src/ui/window.cpp b/doomsday/client/src/ui/window.cpp
index 67e4ca683d..3ec7e2f56f 100644
--- a/doomsday/client/src/ui/window.cpp
+++ b/doomsday/client/src/ui/window.cpp
@@ -253,7 +253,7 @@ struct ddwindow_s
             {
                 geometry.size.width = DisplayMode_Current()->width;
                 geometry.size.height = DisplayMode_Current()->height;
-#if defined MACOSX && defined __CLIENT__
+#if defined MACOSX
                 // Pull the window again over the shield after the mode change.
                 DisplayMode_Native_Raise(Window_NativeHandle(this));
 #endif
@@ -472,7 +472,6 @@ struct ddwindow_s
 
     bool applyAttributes(int* attribs)
     {
-#ifdef __CLIENT__
         LOG_AS("applyAttributes");
 
         bool changed = false;
@@ -572,14 +571,11 @@ struct ddwindow_s
 
         // Seems ok, apply them.
         applyWindowGeometry();
-#endif // __CLIENT__
         return true;
     }
 
     void updateLayout()
     {
-        //qDebug() << "Window::updateLayout:" << widget->geometry() << widget->canvas().geometry();
-
         setFlag(DDWF_MAXIMIZE, widget->isMaximized());
 
         geometry.size.width = widget->width();
@@ -587,37 +583,24 @@ struct ddwindow_s
 
         if(!(flags & DDWF_FULLSCREEN))
         {
-            DEBUG_Message(("Updating current view geometry for window, fetched %i x %i.\n", width(), height()));
+            LOG_DEBUG("Updating current view geometry for window, fetched %i x %i")
+                    << width() << height();
 
             if(!(flags & DDWF_MAXIMIZE) && !isBeingAdjusted())
             {
                 // Update the normal-mode geometry (not fullscreen, not maximized).
                 normalGeometry.size.width = geometry.size.width;
-                DEBUG_Message(("ngw=%i [C]\n", normalGeometry.size.width));
                 normalGeometry.size.height = geometry.size.height;
 
-                DEBUG_Message(("Updating normal view geometry for window, fetched %i x %i.\n", width(), height()));
+                LOG_DEBUG("Updating normal view geometry for window, fetched %i x %i")
+                        << width() << height();
             }
         }
         else
         {
-            DEBUG_Message(("Updating view geometry for fullscreen (%i x %i).\n", width(), height()));
-        }
-
-#ifdef __CLIENT__
-        // Update viewports.
-        R_SetViewGrid(0, 0);
-        if(BusyMode_Active() || UI_IsActive() || !App_GameLoaded())
-        {
-            // Update for busy mode.
-            R_UseViewPort(0);
+            LOG_DEBUG("Updating view geometry for fullscreen (%i x %i)")
+                    << width() << height();
         }
-        R_LoadSystemFonts();
-        if(UI_IsActive())
-        {
-            UI_UpdatePageLayout();
-        }
-#endif
     }
 };
 
@@ -650,7 +633,7 @@ static void updateMainWindowLayout(void)
 
     if(win->flags & DDWF_FULLSCREEN)
     {
-#if defined MACOSX && defined __CLIENT__
+#if defined MACOSX
         // For some interesting reason, we have to scale the window twice in fullscreen mode
         // or the resulting layout won't be correct.
         win->widget->setGeometry(QRect(0, 0, 320, 240));
@@ -1063,7 +1046,7 @@ static void finishMainWindowInit(Canvas& canvas)
     Window* win = canvasToWindow(canvas);
     assert(win == &mainWindow);
 
-#if defined MACOSX && defined __CLIENT__
+#if defined MACOSX
     if(Window_IsFullscreen(win))
     {
         // The window must be manually raised above the shielding window put up by
@@ -1139,9 +1122,8 @@ static void windowWasMoved(CanvasWindow& cw)
     }
 }
 
-static void windowWasResized(Canvas& canvas)
+void Window_UpdateAfterResize(Window *win)
 {
-    Window* win = canvasToWindow(canvas);
     win->assertWindow();
     win->updateLayout();
 }
@@ -1168,7 +1150,6 @@ static Window* createWindow(const char* title)
 
     mainWindow.widget->setCloseFunc(windowIsClosing);
     mainWindow.widget->setMoveFunc(windowWasMoved);
-    mainWindow.widget->canvas().setResizedFunc(windowWasResized);
 
     // Let's see if there are command line options overriding the previous state.
     mainWindow.modifyAccordingToOptions();
@@ -1203,7 +1184,6 @@ void Window_Delete(Window* wnd)
 
     wnd->assertWindow();
     wnd->widget->canvas().setFocusFunc(0);
-    wnd->widget->canvas().setResizedFunc(0);
 
     // Make sure we'll remember the config.
     Window_SaveState(wnd);
@@ -1319,8 +1299,6 @@ void Window_SetDrawFunc(Window* win, void (*drawFunc)(void))
 
 void Window_Draw(Window* win)
 {
-#ifdef __CLIENT__
-
     assert(win);
     assert(win->widget);
 
@@ -1348,8 +1326,6 @@ void Window_Draw(Window* win)
         // Request update at the earliest convenience.
         win->widget->canvas().update();
     }
-
-#endif // __CLIENT__
 }
 
 void Window_Show(Window *wnd, boolean show)
@@ -1555,28 +1531,16 @@ void GL_AssertContextActive(void)
 
 void Window_GLActivate(Window* wnd)
 {
-#ifdef __CLIENT__
-
     wnd->assertWindow();
     wnd->widget->canvas().makeCurrent();
 
     LIBDENG_ASSERT_GL_CONTEXT_ACTIVE();
-
-#else
-    DENG_UNUSED(wnd);
-#endif
 }
 
 void Window_GLDone(Window* wnd)
 {
-#ifdef __CLIENT__
-
     wnd->assertWindow();
     wnd->widget->canvas().doneCurrent();
-
-#else
-    DENG_UNUSED(wnd);
-#endif
 }
 
 QWidget* Window_Widget(Window* wnd)
diff --git a/doomsday/client/src/ui/windowsystem.cpp b/doomsday/client/src/ui/windowsystem.cpp
new file mode 100644
index 0000000000..7a03b7239d
--- /dev/null
+++ b/doomsday/client/src/ui/windowsystem.cpp
@@ -0,0 +1,54 @@
+/** @file windowsystem.cpp  Window management subsystem.
+ *
+ * @authors Copyright (c) 2013 Jaakko Keränen <jaakko.keranen@iki.fi>
+ *
+ * @par License
+ * GPL: http://www.gnu.org/licenses/gpl.html
+ *
+ * <small>This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version. This program is distributed in the hope that it
+ * will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details. You should have received a copy of the GNU
+ * General Public License along with this program; if not, see:
+ * http://www.gnu.org/licenses</small> 
+ */
+
+#include "ui/windowsystem.h"
+#include "ui/window.h"
+
+using namespace de;
+
+DENG2_PIMPL(WindowSystem)
+{
+    /// @todo Creation and ownership of windows belongs here (from window.cpp).
+
+    Instance(Public *i) : Base(i)
+    {}
+};
+
+WindowSystem::WindowSystem() : System(), d(new Instance(this))
+{}
+
+WindowSystem::~WindowSystem()
+{
+    delete d;
+}
+
+bool WindowSystem::processEvent(Event const &event)
+{
+    CanvasWindow *win = Window_CanvasWindow(Window_Main());
+    DENG2_ASSERT(win != 0);
+
+    return win->root().processEvent(event);
+}
+
+void WindowSystem::timeChanged(Clock const &/*clock*/)
+{
+    CanvasWindow *win = Window_CanvasWindow(Window_Main());
+    DENG2_ASSERT(win != 0);
+
+    win->root().update();
+}