Editor Scripting Input Problem OnGUI()

The Mouse down event should only occur once each time the mouse button is pressed down. You are, however, assigning your mouse data too frequently:

Note: If you do not use GUIUtility.hotControl then there is no guarantee that EventType.MouseUp will be triggered.

Example along lines of your question:

 private void CheckInput() {
     // Get a unique ID for your control.
     int controlID = GUIUtility.GetControlID(FocusType.Passive);

     switch (Event.current.GetTypeForControl(controlID)) {
         case EventType.MouseDown:
             // If you have a rectangle for your control then you should
             // perform bounds check here...
             if (Event.current.button == 0) {
                 // Lock on your custom control to prevent interaction with
                 // other controls in your GUI.
                 GUIUtility.hotControl = controlID;
                 Event.current.Use();

                 _isMouseDown = true;
             }
             break;

         case EventType.MouseMove:
         case EventType.MouseDrag:
             // Only update mouse position on move or drag.
             _prevMousePos = _curMousePos;
 
             _curMousePos.x = Event.current.mousePosition.x;
             _curMousePos.y = Event.current.mousePosition.y;
             break;

         case EventType.MouseUp:
             // If this control is currently active.
             if (GUIUtility.hotControl == controlID) {
                 // Release lock on it :)
                 GUIUtility.hotControl = 0;
                 Event.current.Use();

                 _isMouseDown = false;
             }
             break;
     }
 }

Example with GUILayout:

 private static readonly GUIStyle _customStyle;

 private void CheckInput() {
     // Initialise custom style which defines control area of 300x300.
     if (_customStyle == null) {
         _customStyle = new GUIStyle();
         _customStyle.fixedWidth = 300;
         _customStyle.fixedHeight = 300;
     }

     // Get position of control area using layout.
     Rect position = GUILayoutUtility.GetRect(GUIContent.none, _customStyle);
     // Get a unique ID for your control.
     int controlID = GUIUtility.GetControlID(FocusType.Passive, position);

     switch (Event.current.GetTypeForControl(controlID)) {
         case EventType.MouseDown:
             // If you have a rectangle for your control then you should
             // perform bounds check here...
             if (Event.current.button == 0
                     && position.Contains(Event.current.mousePosition)) {
                 // Lock on your custom control to prevent interaction with
                 // other controls in your GUI.
                 GUIUtility.hotControl = controlID;
                 Event.current.Use();

                 _isMouseDown = true;
             }
             break;

         case EventType.MouseMove:
         case EventType.MouseDrag:
             // Only update mouse position on move or drag.
             _prevMousePos = _curMousePos;
 
             _curMousePos.x = Event.current.mousePosition.x;
             _curMousePos.y = Event.current.mousePosition.y;
             break;

         case EventType.MouseUp:
             // If this control is currently active.
             if (GUIUtility.hotControl == controlID) {
                 // Release lock on it :)
                 GUIUtility.hotControl = 0;
                 Event.current.Use();

                 _isMouseDown = false;
             }
             break;
     }
 }