isPressedIn doesn’t really work in Pavlovia, but there is an alternative approach. Unfortunately I think it requires manually programming the javascript in the code component rather than using auto->JS, so set your code type to “Both”.
The key thing is that instead of using mouse.isPressedIn(object), you need to use object.contains(mouse). See here for an example (the rest of the thread is about a mostly unrelated problem): TypeError: Cannot read property 'addChild' of undefined