Notifiy about button state changes

I try to implement a callback in my script to be notified when a button state has changed in modify panel.

This button can be "checked" (blue background) and unchecked, so i was presuming it would be like a checkbox states, but it doesn't work.

Here's the code :

DialogMonitorOPS.unRegisterNotification id:#lookFillet
fn lookFilletCb = 
	filletBtn = windows.getchildhwnd #max "Fillet"
	filletHandle = filletBtn[1]
	local BM_GETSTATE = 0xF2
	local BST_CHECKED = 0x1
	local checkState = windows.sendMessage filletHandle BM_GETSTATE 0 0
	local isChecked = bit.get filletHandle BST_CHECKED
	format "the button was: %\n" isChecked
DialogMonitorOPS.Interactive = false
DialogMonitorOPS.RegisterNotification lookFilletCb id:#lookFillet
DialogMonitorOPS.Enabled = true

The checkState is always equals to 0.
Maybe I could find a way to be notified when the background color of the button has changed, but WM_CTLCOLORBTN gives me always the same value too...

Is there any way to be notified ?

Thanks !