Disabling annoying "select all on focus" for EditorUI.TextArea

DisableSelectAllOnFocus.cs

//Just set EditorGUI.s_SelectAllOnMouseUp (an internal variable) to false every frame / every time you draw. 
//This variable is usually set to true in certain circumstances, so we're overwriting the value.
//I honestly don't know why this feature exists, it's quite annoying and never useful.

//Of course, be sure to cache the field!
selectAllField = typeof(EditorGUI).GetField("s_SelectAllOnMouseUp", BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.Default);
selectAllField.SetValue(null, false);