glFinish() to force the tightest input-to-screen coupling possible

src/RageDisplay_OGL.cpp

@@ -798,12 +798,21 @@ bool RageDisplay_Legacy::BeginFrame()
 
 void RageDisplay_Legacy::EndFrame()
 {
-	glFlush();
-
 	FrameLimitBeforeVsync( g_pWind->GetActualVideoModeParams().rate );
 	g_pWind->SwapBuffers();
 	FrameLimitAfterVsync();
 
+	// Some would advise against glFinish(), ever. Those people don't realize
+	// the degree of freedom GL hosts are permitted in queueing commands.
+	// If left to its own devices, the host could lag behind several frames' worth
+	// of commands.
+	// glFlush() only forces the host to not wait to execute all commands
+	// sent so far; it does NOT block on those commands until they finish.
+	// glFinish() blocks. We WANT to block. Why? This puts the engine state
+	// reflected by the next frame as close as possible to the on-screen
+	// appearance of that frame.
+	glFinish();
+
 	g_pWind->Update();
 
 	RageDisplay::EndFrame();