diff --git a/ClockReceiver/VSyncPredictor.hpp b/ClockReceiver/VSyncPredictor.hpp
index d30f837bc..229a8d636 100644
--- a/ClockReceiver/VSyncPredictor.hpp
+++ b/ClockReceiver/VSyncPredictor.hpp
@@ -10,6 +10,7 @@
 #define VSyncPredictor_hpp
 
 #include "TimeTypes.hpp"
+#include <cassert>
 #include <cmath>
 #include <cstdio>
 
@@ -44,11 +45,22 @@ class VSyncPredictor {
 			frame rate.
 		*/
 		void announce_vsync() {
-			const auto vsync_time = nanos_now();
+			const auto now = nanos_now();
+
 			if(last_vsync_) {
-				vsync_period_.post(vsync_time - last_vsync_);
+				last_vsync_ += frame_duration_;
+				vsync_jitter_.post(last_vsync_ - now);
+				last_vsync_ = (last_vsync_ + now) >> 1;
+			} else {
+				last_vsync_ = now;
 			}
-			last_vsync_ = vsync_time;
+		}
+
+		/*!
+			Sets the frame rate for the target display.
+		*/
+		void set_frame_rate(float rate) {
+			frame_duration_ = Nanos(1'000'000'000.0f / rate);
 		}
 
 		/*!
@@ -75,12 +87,14 @@ class VSyncPredictor {
 			(if those figures are being supplied).
 		*/
 		Nanos suggested_draw_time() {
-			const auto mean = vsync_period_.mean() - redraw_period_.mean() - timer_jitter_.mean();
-			const auto variance = vsync_period_.variance() + redraw_period_.variance() + timer_jitter_.variance();
+			const auto mean = redraw_period_.mean() - timer_jitter_.mean() - vsync_jitter_.mean();
+			const auto variance = redraw_period_.variance() + timer_jitter_.variance() + vsync_jitter_.variance();
 
 			// Permit three standard deviations from the mean, to cover 99.9% of cases.
 			const auto period = mean - Nanos(3.0f * sqrt(float(variance)));
 
+			assert(abs(period) < 10'000'000'000);
+
 			return last_vsync_ + period;
 		}
 
@@ -95,6 +109,7 @@ class VSyncPredictor {
 				}
 
 				void post(Time::Nanos value) {
+					assert(abs(value) < 10'000'000'000);	// 10 seconds is a very liberal maximum.
 					sum_ -= history_[write_pointer_];
 					sum_ += value;
 					history_[write_pointer_] = value;
@@ -110,10 +125,10 @@ class VSyncPredictor {
 					// in whole every time, given the way that the mean mutates.
 					Time::Nanos variance = 0;
 					for(int c = 0; c < 128; ++c) {
-						const auto difference = (history_[c] * 128) - sum_;
-						variance += difference * difference;
+						const auto difference = ((history_[c] * 128) - sum_) / 128;
+						variance += (difference * difference);
 					}
-					return variance / (128 * 128 * 128);
+					return variance / 128;
 				}
 
 			private:
@@ -124,8 +139,9 @@ class VSyncPredictor {
 
 		Nanos redraw_begin_time_ = 0;
 		Nanos last_vsync_ = 0;
+		Nanos frame_duration_ = 1'000'000'000 / 60;
 
-		VarianceCollector vsync_period_{1'000'000'000 / 60};	// 60Hz: seems like a good first guess.
+		VarianceCollector vsync_jitter_{0};
 		VarianceCollector redraw_period_{1'000'000'000 / 60};	// A less convincing first guess.
 		VarianceCollector timer_jitter_{0};						// Seed at 0 in case this feature isn't used by the owner.
 };
diff --git a/OSBindings/Qt/scantargetwidget.cpp b/OSBindings/Qt/scantargetwidget.cpp
index 668cc58a7..b84b62f9e 100644
--- a/OSBindings/Qt/scantargetwidget.cpp
+++ b/OSBindings/Qt/scantargetwidget.cpp
@@ -2,6 +2,7 @@
 
 #include <QDebug>
 #include <QOpenGLContext>
+#include <QScreen>
 #include <QTimer>
 
 #include "../../ClockReceiver/TimeTypes.hpp"
@@ -18,10 +19,18 @@ void ScanTargetWidget::initializeGL() {
 
 void ScanTargetWidget::paintGL() {
 	if(requested_redraw_time_) {
-		vsyncPredictor.add_timer_jitter(Time::nanos_now() - requested_redraw_time_);
+		const auto now = Time::nanos_now();
+		vsyncPredictor.add_timer_jitter(now - requested_redraw_time_);
 		requested_redraw_time_ = 0;
 	}
 
+	const float newOutputScale = float(window()->screen()->devicePixelRatio());
+	if(outputScale != newOutputScale) {
+		outputScale = newOutputScale;
+		resize();
+	}
+	vsyncPredictor.set_frame_rate(float(window()->screen()->refreshRate()));
+
 	glClear(GL_COLOR_BUFFER_BIT);
 
 	// Gmynastics ahoy: if a producer has been specified or previously connected then:
@@ -52,11 +61,8 @@ void ScanTargetWidget::paintGL() {
 		}
 
 		vsyncPredictor.begin_redraw();
-		const int devicePixelRatio = QPaintDevice::devicePixelRatio();
-		const int outputWidth = width()*devicePixelRatio;
-		const int outputHeight = height()*devicePixelRatio;
-		scanTarget->update(outputWidth, outputHeight);
-		scanTarget->draw(outputWidth, outputHeight);
+		scanTarget->update(scaledWidth, scaledHeight);
+		scanTarget->draw(scaledWidth, scaledHeight);
 		glFinish();	// Make sure all costs are properly accounted for in the vsync predictor.
 		vsyncPredictor.end_redraw();
 	}
@@ -79,8 +85,22 @@ void ScanTargetWidget::vsync() {
 }
 
 void ScanTargetWidget::resizeGL(int w, int h) {
-	const int devicePixelRatio = QPaintDevice::devicePixelRatio();
-	glViewport(0, 0, w*devicePixelRatio, h*devicePixelRatio);
+	if(width != w || height != h) {
+		width = w;
+		height = h;
+		resize();
+	}
+}
+
+void ScanTargetWidget::resize() {
+	const int newScaledWidth = int(float(width) * outputScale);
+	const int newScaledHeight = int(float(height) * outputScale);
+
+	if(newScaledWidth != scaledWidth || newScaledHeight != scaledHeight) {
+		scaledWidth = newScaledWidth;
+		scaledHeight = newScaledHeight;
+		glViewport(0, 0, scaledWidth, scaledHeight);
+	}
 }
 
 void ScanTargetWidget::setScanProducer(MachineTypes::ScanProducer *producer) {
diff --git a/OSBindings/Qt/scantargetwidget.h b/OSBindings/Qt/scantargetwidget.h
index 846370a77..d1df0d555 100644
--- a/OSBindings/Qt/scantargetwidget.h
+++ b/OSBindings/Qt/scantargetwidget.h
@@ -38,6 +38,11 @@ class ScanTargetWidget : public QOpenGLWidget
 
 		void setDefaultClearColour();
 
+		int width = 0, height = 0;
+		int scaledWidth = 0, scaledHeight = 0;
+		float outputScale = 1.0f;
+		void resize();
+
 	private slots:
 		void vsync();
 };