I've written about the very unstable screens in my windows. (soon
after I bought this house, Shadow landed outside twice because of
pushing the screens out). I bought these adjustable screen things
that fit in between a set of grooves in the window frame, and so
far it's worked out well. They keep the cats from being able to
lay in the window sill when the windows are open, so they can't
push the screens out. They're only about 8" high, though. They have
perches in front of the windows, so they don't need the sill.

The other night after a thunderstorm it got really cool out, so I
opened up the windows. Soon after, I heard the clear sound of claw
on screen, and looked to see Rhett hooked on one of the living room
window screens, but the screen was completely out of the frame. He
was literally holding the screen in place. I think he was attacking
a bug that was on the outside of the screen. I shudder to think of
him getting out (along with whomever would have followed) if I
wasn't in the room. This is a front window, and in front of my
house is the busy road. I found, ordered, and just received
adjustable screens that are 24" tall instead of the 8" and they fit
in front of the existing screen. Though we've been in the 90*s
lately, so it's not likely the windows will even be open any time
soon. This isn't SPAM, just an endorsement for these screens and
the speedy delivery (I chose shared freight, and it still only took
4 days to receive them).

This is what I got:
http://www.doityourself.com/invt/9406372


--
Cheryl