fix: use SIGWINCH to force fzf to redraw screen

Since I bind my `ctrl-l` to other actions rather than `clear-screen` so
I spot this upsteram bug again
kevinhwang91@bdc2a4e.
`resize` can trigger `clear-screen` in fzf. So this might be a better
way to force `redraw` in linux.

Related:
* ibhagwan/fzf-lua#1871
*
ibhagwan/fzf-lua#1901 (reply in thread)
> * By `resize` there's false positive: e.g. windows manager users may
often resize window manualy
>   * And I'm not sure `SIGWINCH` and fzf's `resize` work correctly on
Windows.
> * By `chan_send`/`feedkeys` we override user's key bindings (although
there's already some `chan_send` hacks in codebase like: toggling
preview (this is handled better by scanning user's fzf bindings
config... but will be weird if user set bindings from
`FZF_DEFAULT_OPTS`), mirroring the `esc` to fzf (this is opt-in by a
hide profile).

Author: phanen

lua/bqf/preview/session.lua

@@ -1,5 +1,6 @@
 local api = vim.api
 local cmd = vim.cmd
+local uv = vim.uv or vim.loop
 
 ---@type BqfPreviewTitle
 local title
@@ -187,8 +188,12 @@ function PreviewSession:display(pwinid, pbufnr, idx, size, handler)
             -- like scrollbar, clear screen manually.
             if hasShowedScrollBar == false and floatwin.showScrollBar == true and utils.has10() and
                 api.nvim_get_mode().mode == 't' then
-                -- ^L = 0x0c
-                api.nvim_feedkeys(('%c'):format(0x0c), 'it', false)
+                if utils.isWindows() then -- ^L = 0x0c
+                    api.nvim_feedkeys(('%c'):format(0x0c), 'it', false)
+                else                      -- SIGWINCH to force fzf redraw on resize
+                    vim.tbl_map(function(pid) uv.kill(pid, 28) end,
+                        vim._os_proc_children(vim.fn.jobpid(vim.bo.channel)))
+                end
             end
         end
     end