MouseLeave of underlying object is fired even when overlapping object is IsHitTestVisible = false
wpf, wpf-controls
Solution
I believe that the Popup control is actually contained within a window, which is why the popup can extend beyond the window bounds in some cases. (It's also why popup transparency is not supported in Silverlight.)
I believe that while the popup control is no longer processing "hits", the container window is, which is why you are losing your button's mouse focus.
I've not tested this, but you might try creating a template for your button and actually declaring the popup as part of the button (rather than below it). This may cause WPF to view the popup control as part of the button and eliminate the problem of losing mouse focus. This works in other scenarios, but I'm not 100% sure how this will work with a Popup.
EDIT: As a side note, the deault WPF tooltip allows you to override the template. I'm not sure what your goals are, but you may find it easier to change the appearance and behavior of the default tooltip than to try to roll your own, as a lot of these sorts of problems have already been solved in the default Tooltip.
Problem
I have created a custom popup to decorate my buttons with animated tooltips. I track Button.MouseEnter for the button to decide when to display the popup. I use Button.MouseLeave to determine when to hide the popup. Problem is Button.MouseLeave is fired prematurely if the popup moves over the mouse cursor (its appearance is animated) despite the fact that I have set IsHitTestVisible = false for the popup and all its visual children. Is this the way WPF is designed to work? I need MouseLeave to only fire when the cursor moves away from the button itself and not be influenced by the popup. Thanks