Check whether a cat can push through, climb around or become trapped in an opening, rather than assuming a small gap is harmless. Ordinary insect screens may not be designed to contain a cat. Use an appropriate secure solution for the window type and follow its installation guidance, seeking professional help where necessary. Keep climbable furniture away from an unsafe opening until it is properly secured. A window perch can be enjoyable, but it also brings the cat close to the barrier, so both the perch and the window need assessment.
Watch for excessive heat or stressful encounters with animals outside. Do not leave a cat unattended beside a newly installed screen until you have checked it carefully. If you are unsure about a particular opening mechanism, ask the manufacturer or an installer rather than improvising a restraint. Check the barrier with the window in each position you normally use, including ventilation settings; a solution tested only with the window closed proves little.
For product comparisons alongside this advice, see Paw-Villian cardboard scratcher and Mouse Chase play set. The details below explain their sizes and uses.
Further reading: rspca.org.uk; cats.org.uk; pdsa.org.uk; pdsa.org.uk.
Paw-Villian cardboard scratcher
Mouse Chase play set
Koda Pod