How to take long screenshots in MIUI 8:
Open your page you wish to take a long screenshot.
1)Long press the volume down (-) and power keys at the same time.
2)Select from toggles button.
3)By Swiping 3 fingers on the Screen
After taking Screenshot an animation will appear on top right corner. Click on the animation icon within 5 seconds.
You’ll see 4 menus named “Scroll”, “Edit”,“Delete” and “Send”. Click on “Scroll”.
Scroll up the content you wish to capture. Or it will automatically scroll. Click on “Done”
The long screenshot is captured now.
You’ve successfully taken the long screen shot. It’ll be saved automatically in the Gallery in Screenshot folder.
Refer the Video: https://youtu.be/XxvEEV4Izds