Polypane

Very good experience with this product, it helps me a lot to detect issues with different displays.

I got to use a free trial of their paid plan as part of the github student developer pack (which ended just recently). I don't think I was savvy enough at the time to use all of their cool features unfortunately (e.g. checking accessibility, social media thumbnail previews, etc). Reading their newsletter updates (which I think are some of the best email content out there), they have so many features I wish I did try them out! At the time I just used Polypane primarily to check how my websites handled different viewport sizes. But then I had wanted to do responsive sizing, so to check there wasn't a weird edge case at an odd viewport size I ended up reverting to just the regular resizing of my browser window ahahaha. Also polypane couldn't handle my framer animations that would appear on entering the viewport that well (some intense flicker that I had to turn on prefersReducedMotion in order to dev the rest of my stuff in polypane). But all-in-all this was an impressive app and I really appreciate the student discount following the year-long free trial.