Try commenting out the graphics.ApplyChanges(). I read an MSDN advisory that said to leave that statement out of initialization code.
Also, I note that you say, “So Game1() is similar initial code of new Monogame project.” Make it identical and see if your secondary problem goes away.
I have set it to 640:480, but after used grapc.IsfullScreen = true and comment grapc.IsfullScreen = true out. my screen show smaller screen too. I dont know why, Only me got this problem?
You need to set the width and height as well as when setting the IsFullScreen flag, when you apply changes and the full screen flag is true it will try and make the full screen to the size set. If your monitor can’t display that resolution at full screen then it will ignore the fullscreen flag. e.g. setting the width & height to both be 500 won’t work in full screen because your monitor can’t display a 500x500 resolution.
To help you out, this is what I have in one of my Game1() constuctors:
The Settings.* you see is just a singleton class I have that holds the settings: something like this:
public static class Settings
{
public static Color BackgroundColor = Color.CornflowerBlue;
public static Difficulty Difficulty = Difficulty.Novice;
public static bool IsFullscreen = false;
public static bool IsMouseVisible = true;
public static int PixelRatio = 2;
public static int ScreenHeight = 256;
public static int ScreenWidth = 144;
}
Hope fully this helps.
BTW I think the default setting for the width/height is 800x640.