How do you maintain angles on vector shapes?

I created a rectangle and dragged the top out further than the bottom so that there’s an angle but when I widen the whole shape together that angle changes based on how much I widen it. I tried uploading a video but “new users” aren’t allowed to upload anything apparently.

If you only widen a shape without also heightening, the proportions will change. You can hold Shift or use the scale tool K to keep the proportions.