Hi @Calm-cpy, thanks for reporting this issue.

Could you specify what you mean by floating window? Is this a vim popup buffer or a neovim buffer? Can you also give an example of what you do to open the floating window?

Didn't reply in time due to time difference. It's vim popup.
About floating window:

"error" is an illegal character. I put two spaces after "error". When the cursor moves to error, my language check plug-in will pop up float windows to display the error message. When pop up the float window, all highlights will fail.

After re entering the insertion mode, the blank character will be highlighted again.
My configuration is as follows：
let g:better_whitespace_enabled=1
let g:strip_whitespace_on_save=0
let g:current_line_whitespace_disabled_hard=0

If the configuration is modified as follows, the highlighting of the blank string will be restored automatically. But there are other problems.
let g:better_whitespace_enabled=1
let g:strip_whitespace_on_save=0
let g:current_line_whitespace_disabled_soft=1

Thank you for your attention

Just tested, the gray highlight in the third figure is the problem of syntax error checking plug-in. It's not about VIM better whitespace

Which plugin do you use for language errors?

dense-analysis/ale

Alright, I seem to be able to reproduce this on vim 8.1. The BufEnter and BufWinEnter do not get fired when the popup opens or closes.

We disable the highlighting from the BufWinLeave which is called from the popup, but for the wrong buffer:

BufWinLeave			Before a buffer is removed from a window.
				Not when it's still visible in another window.
				Also triggered when exiting.  It's triggered
				before BufUnload or BufHidden.
				NOTE: When this autocommand is executed, the
				current buffer "%" may be different from the
				buffer being unloaded "<afile>".
				When exiting and v:dying is 2 or more this
				event is not triggered.

We should either check it is the correct buffer, or remove this event altogether.

@Cimbali thanks for the quick investigation and fix for this, you rock! 😀

@Calm-cpy the fix from Cimbali is merged in now, can you update and give it a try?

The problem has been fixed very well. Now I'm going to use it to enjoy programming. Thanks. 😊