How to keep a window from opening too widely?
It appears that the regulation for the angle of the window is the distance between the mounting screws for the rails in which the brace slides.

The ideal work-around would entail removing the bottom screw from each side and inserting a metal shim that fits within the track below the slider. Return the screw and the slider cannot drop far enough to open the window to cat-head-width. CAUTION: cats can pass through holes far smaller than you might think!

One aspect of this method is that the sliding mechanism, including the rail, has to be accessible when the window is closed to the maximum desired opening.

But wait, there's a work-around to that as well.

The shim that would have been inserted between the bottom screw and the slider now has a slot to allow it to pass around the bottom screw. It is of sufficient length to reach to the bottom of the window frame and can be slipped into place even if the window opens only a few inches.



Depending on the width of the pivot assembly, one may be able to purchase a piece of aluminum or steel of sufficient width to work in this manner. The Home Depot has a materials section with solid aluminum rod and flat as well as solid steel rod and flat. Aluminum is easier to work and should hold up well enough if one does not slam things together.

You'd need a piece thin enough to slip under the inward facing lips with sufficient width to engage as much of the lip area as possible. Even if the thinnest piece you find is too thick, you could file or sand away material to get it to fit in place.



In the image above, the top represents the view from above if the flat aluminum is thin enough to fit in the groove/lips. The bottom is the answer if the piece is too thick, and has the corners filed/sanded away to provide placement.

All of this should work with the recently revealed "bumps" that provide limitation of travel of the sliding mechanism. The bumps are going to pose a problem with placing a single spacer above the screw, but the single spacer above the screw can be of the right hand side of my original drawing with a hole for the screw to engage.