Issue:
When adding a function to GUI.Rectangle.onPointerUpObservable the event is not firing when using chrome on desktop until the mouse leaves the canvas.
Example:
https://www.babylonjs-playground.com/#3VMTI9#136
It work’s fine on mobile chrome.
Bug is present on: Windows 10 x64, Google Chrome 79.0.3945.117 x64,
Bug is not present on: OnePlus 5T, Google Chrome App 79.0.3945.116
Btw. I’ts very irritating that such a fundamental basic feature as drag and drop is not part of the GUI framework 
Hello and welcome!
That is a very mean way to ask for help if you want my thoughts
To your question: just add dragBox.isPointerBlocker = true; so your Rectangle will receive the events (off by default for controls with no user interaction)
This wasn’t meant to be mean.
Thank you it works 
But this gets me thinking: why is it firing on mobile when it isn’t blocking?
Why is the “PointerDown” event firing when it isn’t blocking?
Down pointer is just bubbling up (so you are lucky) as it is the top control 