Secondary buffer window restore glitch

[parkovski]

• Your Windows build number: (Type ver at a Windows Command Prompt)
  17682

• What you're doing and what's happening: (Copy & paste specific commands and their output, or include screen shots)
  Run vim -u NONE -i NONE "+set tgc" (needs a recent Windows vim). Maximize the window and then restore it. The buffer will have some visual glitches - the some of the tildes are in random places on the screen. This is a minimal repro, and only happens with termguicolors set (e.g. using ANSI colors in the secondary buffer). The issue is more apparent though if you have some vim plugins installed:
  

After typing a couple lines with my vimrc:

If you're using no vimrc, it helps to see the issue if you type a couple lines of text and/or set number first. The buffer gets pushed about a third of the way down the screen, and the bottom third appears on top.

• What's wrong / what should be happening instead:
  The buffer shouldn't be glitchy.