Missing mouse up event from different canvas in smartGWT

I have two canvases located to each other. One is supposed to be a kind of workspace you can add items to, select and rearange them. The other one is just a property view.

What I want to do is to be able to draw a rectrangle on the workspace. As long as the user holds down the mouse button the rectangle will show up. If he releases the button all items that are beneath the rectangle will be selected. This currently works good with the MouseDown, MouseStillDown und MouseUp events. I'm drawing another rectangle shaed canvas on the workspace which will be transformed on every MouseStillDown event and the selection will occur on the MouseUp event. My problem is, that if the user hold down the mouse button and moves it to the property canvas and then releases the button the MouseUp event from the workspace isn't called. Neither is the one from the property since it's missing a MouseDown event. So 开发者_如何学JAVAif the user releases the button there the selection won't work and the rectangle stays in the workspace.

Is there an oppurtunity to somehow avoid this? Or is there a better way to determine the area the user selected with it's mouse while holding down the left mouse button?