Problem with IPointerDown?

I was following https://unity3d.com/learn/tutorials/modules/intermediate/live-training-archive/panels-panes-windows tutorial to make a draggable panel but I could not get it to work. I am currently using version 5.4.03f of Unity Pro.

I am looking for an explanation of the following behavior:

My Steps:
Create new project 2D
Add Canvas to scene (leave defaults)
Add Panel to Canvas
Create Script called FocusPanel
Add Code

public class FocusPanel : MonoBehaviour, IPointerDownHandler
{
private RectTransform panel;
void Awake()
{
panel = GetComponent<RectTransform>();
}

public void OnPointerDown(PointerEventData data)
{
Debug.Log("Pointer Down!");
panel.SetAsLastSibling();
}
}

Add script to Panel
Run game
Click on panel
See log (Great!)
Stop game
Hold shift and resize panel to be something like half the original size, in either or both directions (like he does in the tutorial)
Run game
Click on Panel
No log (!What??)

Why does OnPointerDown not get called after I resize the panel?
If I resize the panel back to being just a little smaller than the Canvas it works again, though it seems to fire even if I click on the space outside of the panel. It seems that once the panel is about 2/3 the size of the canvas the event no longer fires.

 

That code Works correctly in my computer.

IPointerDownHandler need EventSystem component to work.

 

I have an EventSystem, it currently sits above the canvas in my scene. I was wondering if something got messed up because I did the recent update to 5.3.1 but then had to downgrade.

 

I figured out the issue. It seems that if I have un-docked windows there is a chance that the input events stop firing.

Example:
Create a second Game tab
Run game
In tab 1 click and see log
In tab 2 click and see log
Pull tab 2 out of editor to standalone window.
Click in tab 1, may see log
Click in tab 2 (now windowed), may see log
Drag tab 2 back into editor
Log will now appear from both tabs.

Only 1 of the two windows will fire the events even though they are both set to the same display. I have also seen that separate windows that are looking at different displays can cause the same effect.