How to take long screenshots in MIUI 8:
Open your desired page or content you wish to take a long screenshot. Long press the volume down (-) and power keys at the same time.
Right after the long press and animation will appear on top right corner. Click on the animation icon within 5 seconds.
You’ll see three menus named “Extend”, “Edit” an “Send”. Click on Extend.
Scroll up the content you wish to capture. Press “Stop” when you’re done.
The long screenshot is captured now. You’ll see two menus named “Edit”, “Send”. You can easily edit or send your screenshot from this two menu.
You’ve successfully taken the long screen shot. It’ll be saved automatically in the Gallery in Screenshot folder.
[Tutorial] MIUI 8: How To Use The New Long Screenshot Feature