Tooltip does not work sometimes (mouse events are missing)


I wrote a connector which uses a canvas as UI element. I also added a tooltip, which is only visible when the cursor is in a special region (the connector says, there is a tooltip (than the Vaadin framework adds a mouselistener for the tooltip etc.) and I added a mouse listener on client side which analizes the mouse position during mouse move and if it is in right position I set the tooltip or I set it to null in the corresponding state). It works fine but sometimes it does not work and I don’t know why. My mousemovelistener works and does its job. But the mouselistener of the tooltip which is registered as DOM-listener sometimes gets no more mouse events (I do not stop event propagation or something like that). And because of that the tooltip is not shown.
When I move the mouse out of the canvas and back on canvas it works again. Does anyone has an idea what could be the problem? Till now I have no idea. I also don’t see any error messages or something like that.

Thanks in advance, Philipp