The mouse up event is being used by the Scene View to pick objects. To register a user control for mouse button events you need to interface with the hotControl mechanisms.

using UnityEngine;
using UnityEditor;

public class Test : EditorWindow
{
[MenuItem("Tools/EasyPlace &%g")]
public static void Snapper() => GetWindow<Test>("EasyPlace");

void OnEnable() => SceneView.duringSceneGui += DuringSceneGui;
void OnDisable() => SceneView.duringSceneGui -= DuringSceneGui;

static void DuringSceneGui(SceneView sceneView)
{
Event evt = Event.current;
int id = GUIUtility.GetControlID(FocusType.Passive);

// Mouse button 0 is used by the scene view to pick GameObjects. In order to capture mouse events for the left
// button you will need to make the GUI system aware of your intent to use the event.
switch (evt.type)
{
case EventType.Layout:
case EventType.MouseMove:
// AddDefaultControl means that if no other control is selected, this will be chosen as the fallback.
// This allows things like the translate handle and buttons to function.
HandleUtility.AddDefaultControl(id);
break;

case EventType.MouseDown:
if (evt.button == 0 && HandleUtility.nearestControl == id)
{
Debug.Log("mouse button 0 down");
// Setting the hotControl tells the Scene View that this mouse down/up event cannot be considered
// a picking action because the event is in use.
GUIUtility.hotControl = id;
evt.Use();
}
break;
case EventType.MouseUp:
{
if (evt.button == 0 && GUIUtility.hotControl == id)
{
Debug.Log("mouse button 0 up");
GUIUtility.hotControl = 0;
evt.Use();
}

break;
}
}
}
}