FullscreenPlayMode

using UnityEditor;
using UnityEngine;
using System.Collections;
using System.Reflection;
using System;

/// <summary>
/// From https://www.reddit.com/r/Unity3D/comments/2lymim/full_full_screen_on_play_script_freebie_for/
/// and https://answers.unity.com/questions/956123/add-and-select-game-view-resolution.html
/// </summary>
[InitializeOnLoad]
public class FullscreenPlayMode : MonoBehaviour {
    public static bool fullscreenEnabled = true;

    //The size of the toolbar above the game view, excluding the OS border.
    private static int tabHeight = 22;
    static object gameViewSizesInstance;
    static MethodInfo getGroup;

    static FullscreenPlayMode() {
        if (!fullscreenEnabled) return;
        EditorApplication.playModeStateChanged -= CheckPlayModeState;
        EditorApplication.playModeStateChanged += CheckPlayModeState;

        var sizesType = typeof(Editor).Assembly.GetType("UnityEditor.GameViewSizes");
        var singleType = typeof(ScriptableSingleton<>).MakeGenericType(sizesType);
        var instanceProp = singleType.GetProperty("instance");
        getGroup = sizesType.GetMethod("GetGroup");
        gameViewSizesInstance = instanceProp.GetValue(null, null);
    }

    /// <summary>
    /// Only called with EnteredPlayMode and EnteredEditMode, never Exit for some reason in 2017.3.0f3
    /// </summary>
    static void CheckPlayModeState(PlayModeStateChange state) {
        if (!fullscreenEnabled) return;
        if (state == PlayModeStateChange.EnteredPlayMode) {
            FullScreenGameWindow();
        } else if (state == PlayModeStateChange.EnteredEditMode) {
            CloseGameWindow();
        }
    }

    static EditorWindow GetMainGameView() {
        EditorApplication.ExecuteMenuItem("Window/Game");

        System.Type T = System.Type.GetType("UnityEditor.GameView,UnityEditor");
        System.Reflection.MethodInfo GetMainGameView = T.GetMethod("GetMainGameView", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);
        System.Object Res = GetMainGameView.Invoke(null, null);
        return (EditorWindow)Res;
    }

    static void FullScreenGameWindow() {
        EditorWindow gameView = GetMainGameView();

        //gameView.title = "Game (Stereo)";
        Rect newPos = new Rect(0, 0 - tabHeight, Screen.currentResolution.width, Screen.currentResolution.height + tabHeight);

        // move down to second monitor beneath main one
        newPos.y += 1080;

        gameView.position = newPos;
        gameView.minSize = new Vector2(Screen.currentResolution.width, Screen.currentResolution.height + tabHeight);
        gameView.maxSize = gameView.minSize;
        gameView.position = newPos;

        // since camera may have moved, re-align the scene view to it
        NWUtils.RunLater(NWUtils.AlignSceneToCamera, 0.5f, true);
    }

    static void CloseGameWindow() {
        EditorWindow gameView = GetMainGameView();
        gameView.Close();

        Type gameViewType = Type.GetType("UnityEditor.GameView,UnityEditor.dll");
        Type[] desiredDockNextTo = new Type[] { typeof(UnityEditor.SceneView) };

        MethodInfo getWindow = typeof(EditorWindow).GetMethod("GetWindow", new Type[] { typeof(string), typeof(bool), desiredDockNextTo.GetType() });
        MethodInfo getWindowGameView = getWindow.MakeGenericMethod(gameViewType);
        getWindowGameView.Invoke(null, new object[] { "Game", false, desiredDockNextTo });

        // set the game screen size back to 16:9
        SetWidescreen();

        // set focus back on Scene tab
        EditorWindow.FocusWindowIfItsOpen(typeof(UnityEditor.SceneView));

        // since camera may have moved, re-align the scene view to it
        NWUtils.AlignSceneToCamera();
    }

    public static void MaximizeGameWindow() {
        var assembly = Assembly.GetAssembly(typeof(Editor));
        var type = assembly.GetType("UnityEditor.GameView");
        var property = type.GetProperty("maximized");
        var window = EditorWindow.GetWindow(type);
        var maximized = (bool)property.GetValue(window, null);
        property.SetValue(window, (object)(!maximized), null);
    }

    public static void SetWidescreen() {
        SetSize(FindSize(GameViewSizeGroupType.Standalone, "16:9"));
    }

    public static void SetSize(int index) {
        var gvWndType = typeof(Editor).Assembly.GetType("UnityEditor.GameView");
        var selectedSizeIndexProp = gvWndType.GetProperty("selectedSizeIndex",
                BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
        var gvWnd = EditorWindow.GetWindow(gvWndType);
        selectedSizeIndexProp.SetValue(gvWnd, index, null);
    }

    public static int FindSize(GameViewSizeGroupType sizeGroupType, string text) {
        // GameViewSizes group = gameViewSizesInstance.GetGroup(sizeGroupType);
        // string[] texts = group.GetDisplayTexts();
        // for loop...

        var group = GetGroup(sizeGroupType);
        var getDisplayTexts = group.GetType().GetMethod("GetDisplayTexts");
        var displayTexts = getDisplayTexts.Invoke(group, null) as string[];
        for (int i = 0; i < displayTexts.Length; i++) {
            string display = displayTexts[i];
            // the text we get is "Name (W:H)" if the size has a name, or just "W:H" e.g. 16:9
            // so if we're querying a custom size text we substring to only get the name
            // You could see the outputs by just logging
            // Debug.Log(display);
            int pren = display.IndexOf('(');
            if (pren != -1)
                display = display.Substring(0, pren - 1); // -1 to remove the space that's before the prens. This is very implementation-depdenent
            if (display == text)
                return i;
        }
        return -1;
    }
    static object GetGroup(GameViewSizeGroupType type) {
        return getGroup.Invoke(gameViewSizesInstance, new object[] { (int)type });
    }
}