fix(ui): exclude unfocusable windows from 'complete' "w" completion

Problem:  As in f85bc41, assume unfocusable windows to be UI windows
          whose buffer content is unexpectedly included in 'complete'
          "w" completion.
Solution: Exclude unfocusable windows when looping over windows.
(cherry picked from commit d01b2611a6)

runtime/doc/windows.txt

@@ -69,8 +69,9 @@ If a window is focusable, it is part of the "navigation stack", that is,
 editor commands such as :windo, |CTRL-W|, etc., will consider the window as
 one that can be made the "current window". A non-focusable window will be
 skipped by such commands (though it can be explicitly focused by
-|nvim_set_current_win()|). Non-focusable windows are not listed by |:tabs|, and
-are not counted by the default 'tabline'.
+|nvim_set_current_win()|). Non-focusable windows are not listed by |:tabs|,
+or counted by the default 'tabline'. Their buffer content is not included
+in 'complete' "w" completion.
 
 Windows (especially floating windows) can have many other |api-win_config|
 properties such as "hide" and "fixed" which also affect behavior.

src/nvim/insexpand.c

@@ -2460,8 +2460,8 @@ static buf_T *ins_compl_next_buf(buf_T *buf, int flag)
     while (true) {
       // Move to next window (wrap to first window if at the end)
       wp = (wp->w_next != NULL) ? wp->w_next : firstwin;
-      // Break if we're back at start or found an unscanned buffer
-      if (wp == curwin || !wp->w_buffer->b_scanned) {
+      // Break if we're back at start or found an unscanned buffer (in a focusable window)
+      if (wp == curwin || (!wp->w_buffer->b_scanned && wp->w_config.focusable)) {
         break;
       }
     }

test/functional/editor/completion_spec.lua

@@ -1351,4 +1351,26 @@ describe('completion', function()
       eq('completeopt option does not include popup', api.nvim_get_var('err_msg'))
     end)
   end)
+
+  it([[does not include buffer from non-focusable window for 'complete' "w"]], function()
+    local buf = api.nvim_create_buf(false, true)
+    local cfg = { focusable = false, relative = 'win', bufpos = { 1, 0 }, width = 1, height = 1 }
+    local win = api.nvim_open_win(buf, false, cfg)
+    api.nvim_buf_set_lines(buf, 0, -1, false, { 'foo' })
+    feed('i<C-N>')
+    screen:expect([[
+      ^                                                            |
+      {4:f}{1:                                                           }|
+      {1:~                                                           }|*5
+      {5:-- Keyword completion (^N^P) }{9:Pattern not found}              |
+    ]])
+    api.nvim_win_set_config(win, { focusable = true })
+    feed('<Esc>i<C-N>')
+    screen:expect([[
+      foo^                                                         |
+      {4:f}{1:                                                           }|
+      {1:~                                                           }|*5
+      {5:-- Keyword completion (^N^P) The only match}                 |
+    ]])
+  end)
 end)