The event can’t be captured while the context menu is out of panel’s range

Question

I'm developing a PowerPoint plugin with VSTO now, I use WPF as a PowerPoint's panel, like this: http://www.freeimagehosting.net/uploads/th.5a91c11d17.png

of curse,that's a built-in panel in PowerPoint.

but in my custom panel,

If I click at A, the mouse event can be captured,

but click at B, the mouse event can not be captured.

any suggestion ?