[MC-33383] Lag while taking screenshot with F2 in singleplayer

The bug

While taking a screenshot using the in-game F2 key, the game freezes for a second and does not take an accurate screenshot.

Code analysis

The reason why this is happening is because saving the screenshot is not done by a seperate thread. This means the lag time is determined by the writing speed of your harddrive.
Another reason which has probably a rather small impact is that in case frame buffering is enabled Minecraft iterates over all pixels in the screenshot and sets them in a BufferedImage. I do not see any advantage of this.
Here is a "fix" that solves this problem, however the ScreenshotSavingThread is only used as a local variable to demonstrate that it solves the problem. It should be rather in the class itself and also set the error message in case it fails:
saveScreenshot(File p_148259_0_, String p_148259_1_, int p_148259_2_, int p_148259_3_, Framebuffer p_148259_4_) method of the net.minecraft.util.ScreenShotHelper class (MCP 1.8 names)

saveScreenshot(File p_148259_0_, String p_148259_1_, int p_148259_2_, int p_148259_3_, Framebuffer p_148259_4_)

// Is there any advantage from this?
if (OpenGlHelper.isFramebufferEnabled())
{
	var7 = new BufferedImage(p_148259_4_.framebufferWidth, p_148259_4_.framebufferHeight, 1);
	int var8 = p_148259_4_.framebufferTextureHeight - p_148259_4_.framebufferHeight;

	for (int var9 = var8; var9 < p_148259_4_.framebufferTextureHeight; ++var9)
	{
		for (int var10 = 0; var10 < p_148259_4_.framebufferWidth; ++var10)
		{
			var7.setRGB(var10, var9 - var8, pixelValues[var9 * p_148259_4_.framebufferTextureWidth + var10]);
		}
	}
}
else
{
	var7 = new BufferedImage(p_148259_2_, p_148259_3_, 1);
	var7.setRGB(0, 0, p_148259_2_, p_148259_3_, pixelValues, 0, p_148259_2_);
}

File var12;

if (p_148259_1_ == null)
{
	var12 = getTimestampedPNGFileForDirectory(var5);
}
else
{
	var12 = new File(var5, p_148259_1_);
}

// Create an empty file to make sure a ScreenshotSavingThreads do not override it by accident
var12.createNewFile();

//ImageIO.write(var7, "png", var12);
class ScreenshotSavingThread extends Thread {
	private final BufferedImage bufferedImage;
	private final File file;
	
	private ScreenshotSavingThread(BufferedImage bufferedImage, File file) {
		this.bufferedImage = bufferedImage;
		this.file = file;
	}
	
	@Override
	public void run() {
		try {
			ImageIO.write(bufferedImage, "png", file);
		} catch (IOException e) {
			// TODO Print the error message in chat and in the log
		}
	}
}
new ScreenshotSavingThread(var7, var12).start();

Flaws with this design

Stopping Minecraft could cause half-finished screenshots (unlikely); thread could be non-daemon
Taking screenshots to often results in a OutOfMemoryError (unlikely, that would need with 500KB per screenshot and 500MB allocated RAM about 1000 ScreenshotSavingThreads running at once)
Reporting error or success would have to be synchronized and would appear delayed which could be confusing