mouse leave event won't stop triggering when...

...you hold the mouse button down and leave a picture window.

When you're not holding a button down, it triggers just once, so
i'm assuming this is normal behavior.

Confirmed on Vista Ultimate 32bit, 6.32

Code:

menu @test {
  leave: { echo -s test $rand(1,4444444) }
}

/window -dp @test and hold mouse button down when leaving window

This is intended behavior, and pretty useful at that. It lets you drag on a virtual area larger than the actual window.

Indeed, it's been like that forever. One example of its use is custom resizing routines (eg resizing a picture window while keeping a fixed width-to-height ratio). Without it they wouldn't work too well.