Fixes: drop any processing backlog, try not to allow an Electron document to close mid-draw, perform a frame grab even if the emulated machine is over-processing, really really don't create a CRT until it's safe.

Machines/Electron/Electron.cpp

@@ -45,7 +45,7 @@ Machine::Machine() :
 	_audioOutputPositionError(0),
 	_current_pixel_line(-1),
 	_use_fast_tape_hack(false),
-	_crt(std::unique_ptr<Outputs::CRT::CRT>(new Outputs::CRT::CRT(crt_cycles_per_line, 8, Outputs::CRT::DisplayType::PAL50, 1)))
+	_crt(nullptr)
 {
 	memset(_key_states, 0, sizeof(_key_states));
 	memset(_palette, 0xf, sizeof(_palette));
@@ -57,6 +57,7 @@ Machine::Machine() :
 
 void Machine::setup_output()
 {
+	_crt = std::unique_ptr<Outputs::CRT::CRT>(new Outputs::CRT::CRT(crt_cycles_per_line, 8, Outputs::CRT::DisplayType::PAL50, 1));
 	_crt->set_rgb_sampling_function(
 		"vec4 rgb_sample(vec2 coordinate)"
 		"{"

OSBindings/Mac/Clock Signal/Documents/ElectronDocument.swift

@@ -58,11 +58,14 @@ class ElectronDocument: MachineDocument {
 	}
 
 	lazy var actionLock = NSLock()
+	lazy var drawLock = NSLock()
 	override func close() {
 		actionLock.lock()
+		drawLock.lock()
 		openGLView.invalidate()
 		openGLView.openGLContext!.makeCurrentContext()
 		actionLock.unlock()
+		drawLock.unlock()
 
 		super.close()
 	}
@@ -76,7 +79,10 @@ class ElectronDocument: MachineDocument {
 	}
 
 	override func openGLView(view: CSOpenGLView, drawViewOnlyIfDirty onlyIfDirty: Bool) {
-		electron.drawViewForPixelSize(view.backingSize, onlyIfDirty: onlyIfDirty)
+		if drawLock.tryLock() {
+			electron.drawViewForPixelSize(view.backingSize, onlyIfDirty: onlyIfDirty)
+			drawLock.unlock()
+		}
 	}
 
 	// MARK: CSOpenGLViewResponderDelegate

OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift

@@ -41,7 +41,12 @@ class MachineDocument: NSDocument, CSOpenGLViewDelegate, CSOpenGLViewResponderDe
 		let cycleCount = cycleCountLow + cycleCountHigh
 		if let lastCycleCount = lastCycleCount {
 			let elapsedTime = cycleCount - lastCycleCount
-			runForNumberOfCycles(Int32(elapsedTime))
+			// if the emulation has fallen too far behind then silently swallow the request;
+			// some actions — e.g. the host computer waking after sleep — may give us a
+			// prohibitive backlog
+			if elapsedTime < intendedCyclesPerSecond / 25 {
+				runForNumberOfCycles(Int32(elapsedTime))
+			}
 		}
 		lastCycleCount = cycleCount
 	}

OSBindings/Mac/Clock Signal/Views/CSOpenGLView.m

@@ -62,6 +62,10 @@ static CVReturn DisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeSt
 			OSAtomicTestAndClear(activityMask, &_updateIsOngoing);
 		});
 	}
+	else
+	{
+		[self drawViewOnlyIfDirty:YES];
+	}
 }
 
 - (void)invalidate