[RDY] Fix incsearch highlighting disappearing when redrawing from timer

[gelguy]

Redrawing from a timer clears temporary highlights when a floating window needs to be redrawn. Temporary highlights here refers to highlights applied when performing a / search when incsearch is active. This issue is not unique to floating windows, any action that would invalidate highlighting will have the same issue.

This PR only fixes the issue for the Search highlights, IncSearch needs a separate fix.

Minimal init.vim:

let s:height = 1
let s:win = nvim_open_win(0, 0,
      \ {'relative': 'editor', 'height': 1, 'width': 1, 'row': 0, 'col': 0})

function! Foo() abort
  let s:height = 3 - s:height
  call nvim_win_set_config(s:win, {'height': s:height})
  redraw
endfunction

autocmd CmdlineChanged * call timer_start(0, {-> Foo()})

Open a file and do a search to match something in the file. The search highlights will either flash temporarily on the screen or not be appear.

This is somewhat similar to vim/vim#3694, but the redrawing mechanism between Vim and Neovim has diverged quite a bit. The gist behind both issues are the same - (some) temporary highlights are not preserved when redrawing from a timer.

[gelguy]

Closing since code is outdated.