feat(api): allow open_win/win_set_buf in the cmdwin in some cases

Problem: As discussed on Matrix, there was some interest in having
`nvim_open_win` again be able to open floats in the cmdwin (e.g: displaying a
hover doc related to what's in the cmdwin). After #23228, this was disallowed.

Solution: Allow `nvim_open_win` in the cmdwin as long as `!enter` and
`buffer != curbuf` (the former can cause all sorts of issues, and the latter
can crash Nvim after closing cmdwin). Also allow `nvim_win_set_buf` in a similar
fashion.

Note that we're not *entirely* sure if this is 100% safe (cmdwin is a
global-state-using-main-loop-calling beast), but this seems to work OK..?

Also:
  - Check the buffer argument of `nvim_open_win` earlier, and abort if it's
    invalid (it used to still open a window in this case).

  - Untranslate `e_cmdwin` errors in the API (other errors in the API are not
    translated: although not detailed in the API contract yet, errors are
    supposed to be stable).

1 files changed, 41 insertions, 0 deletions

diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 19511f30f1..e7e767817b 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -43,6 +43,22 @@ describe('API/win', function()
       eq('Invalid buffer id: 23', pcall_err(window, 'set_buf', nvim('get_current_win'), 23))
       eq('Invalid window id: 23', pcall_err(window, 'set_buf', 23, nvim('get_current_buf')))
     end)
+
+    it('disallowed in cmdwin if win=curwin or buf=curbuf', function()
+      local new_buf = meths.create_buf(true, true)
+      local new_win = meths.open_win(new_buf, false, {
+        relative='editor', row=10, col=10, width=50, height=10,
+      })
+      feed('q:')
+      eq('E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+         pcall_err(meths.win_set_buf, 0, new_buf))
+      eq('E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+         pcall_err(meths.win_set_buf, new_win, 0))
+
+      local next_buf = meths.create_buf(true, true)
+      meths.win_set_buf(new_win, next_buf)
+      eq(next_buf, meths.win_get_buf(new_win))
+    end)
   end)
 
   describe('{get,set}_cursor', function()
@@ -821,6 +837,31 @@ describe('API/win', function()
       })
       eq(1, funcs.exists('g:fired'))
     end)
+
+    it('disallowed in cmdwin if enter=true or buf=curbuf', function()
+      local new_buf = meths.create_buf(true, true)
+      feed('q:')
+      eq('E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+         pcall_err(meths.open_win, new_buf, true, {
+           relative='editor', row=5, col=5, width=5, height=5,
+         }))
+      eq('E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+         pcall_err(meths.open_win, 0, false, {
+           relative='editor', row=5, col=5, width=5, height=5,
+         }))
+
+      eq(new_buf, meths.win_get_buf(meths.open_win(new_buf, false, {
+           relative='editor', row=5, col=5, width=5, height=5,
+         })))
+    end)
+
+    it('aborts if buffer is invalid', function()
+      local wins_before = meths.list_wins()
+      eq('Invalid buffer id: 1337', pcall_err(meths.open_win, 1337, false, {
+           relative='editor', row=5, col=5, width=5, height=5,
+         }))
+      eq(wins_before, meths.list_wins())
+    end)
   end)
 
   describe('get_config', function()