input: Do not set high-bit; preserve ALT modifier.

Background: Vim internally prefers to represent ALT/META chords as
single-byte keys, by setting the high bit of the key byte.
extract_modifiers() _discards_ the meta/alt modifier, but we need it for
libvterm and libtermkey.

Closes #2440
Closes #3727
Closes #2017
References #2277
References #2254

https://github.com/neovim/neovim/issues/2017#issuecomment-140423557
> We [not libtermkey] are setting the high bit for some reason

https://github.com/neovim/neovim/issues/176#issuecomment-77834715
> libvtermkey requires the leading esc to parse alt/meta

https://github.com/neovim/neovim/pull/3246#issuecomment-136328450
> A program could do better than the current logic on some terminals, by
> asking for pure 8bit mode (S8C1T) and then immediately querying the
> mode again. If the result comes back as an 8bit single-byte CSI, then
> it can presume the mode setting was successful, and now the ESC prefix
> byte won't be seen in multibyte sequences; only as an Alt- prefix or
> a real Escape key. On such a terminal, it could therefore avoid
> needing to use that waiting timeout.

1 files changed, 20 insertions, 1 deletions

diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 14700a2622..838d05a6df 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -53,7 +53,7 @@ describe('tui', function()
     ]])
   end)
 
-  it('interprets leading esc byte as the alt modifier', function()
+  it('interprets leading <Esc> byte as ALT modifier in normal-mode', function()
     local keys = 'dfghjkl'
     for c in keys:gmatch('.') do
       execute('nnoremap <a-'..c..'> ialt-'..c..'<cr><esc>')
@@ -80,6 +80,25 @@ describe('tui', function()
     ]])
   end)
 
+  it('does not mangle unmapped ALT-key chord', function()
+    -- Vim represents ALT/META by setting the "high bit" of the modified key;
+    -- we do _not_. #3982
+    --
+    -- Example: for input ALT+j:
+    --    * Vim (Nvim prior to #3982) sets high-bit, inserts "ê".
+    --    * Nvim (after #3982) inserts "j".
+    feed('i\x1bj')
+    screen:expect([[
+      j{1: }                                                |
+      ~                                                 |
+      ~                                                 |
+      ~                                                 |
+      [No Name] [+]                                     |
+      -- INSERT --                                      |
+      -- TERMINAL --                                    |
+    ]])
+  end)
+
   it('accepts ascii control sequences', function()
     feed('i')
     feed('\x16\x07') -- ctrl+g