• Your Windows build number: (Type ver at a Windows Command Prompt)
  Microsoft Windows [Version 10.0.18362.86]

• What you're doing and what's happening: (Copy & paste specific commands and their output, or include screen shots)
  In conhost or Windows Terminal, using both Vim.exe and WSL Vim, background color rendering becomes broken if a background color >16 or true colors is used.

With an example file, e.g. https://github.com/GilbertsHub/web-utils/blob/master/cgibinIntercept.sh:

vim -u NONE cgibinIntercept.sh +"hi Normal ctermbg=17" # for Windows: +"set t_Co=256"
# scroll down with CTRL-D

In general it is much easier to reproduce in WSL than Vim.exe, because it doesn't seem to occur after the initial scroll on Vim.exe and the effects are a bit different as described below. I can reproduce with any combination of conhost/terminal and Vim 8.1.883 on Ubuntu/8.0 on Debian/8.1 on Win32. Does not reproduce with neovim (v0.4.0-430-g8698830cb).

A few cases of example behavior:

• Debian with conhost, Vim from official repository

$ sudo apt install vim
$ vim --version
VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Sep 30 2017 18:21:38)
Included patches: 1-197, 322, 377-378, 550, 703, 706-707

Same behavior in the new terminal.

• Vim.exe on conhost cmd.exe installed from https://github.com/vim/vim-win32-installer/releases/download/v8.1.1099/gvim_8.1.1099_x64.exe
  
  Background color is wrong on the text part but correct otherwise (purple, although this seems to depend on active colorscheme) but is fixed after a Ctrl-L redraw.

• Vim.exe on Terminal cmd.exe, same as above
  
  Background color is again wrong, but the background highlight completely disappears after a Ctrl-L redraw.

• What's wrong / what should be happening instead:
  Background rendered the whole width of the window, e.g. in wsltty:
  

The effect is very similar to the issue #375, however that one happens with bg color set to <16 (darkBlue). However I cannot reproduce that issue anymore, including the terminal bg change.

The issues for Win32 and Linux versions might be separate, since their effects are pretty different as described above.

I also haven't been able to reproduce this using VT sequences, but I hope the bug report is still useful.