There are many alternatives to PTgui for Mac if you are looking for a replacement. The best Mac alternative is Affinity Photo. It's not free, so if you're looking for a free alternative, you could try Hugin or Stitch Panorama. If that doesn't suit you, our users have ranked more than 25 alternatives to PTgui and 15 are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to PTgui are Adobe Photoshop Lightroom Classic (Paid), AutoPano (Paid), PanoramaStudio (Paid) and AutoStitch Panorama (Freemium).
PhotoStitcher is a powerful panorama stitcher software that enables users to merge multiple images/photos/screenshots together. It could also help you to create one single and long screenshot that extends beyond the screen's limit with a few clicks.