fix(input): do no reinterpret mouse keys with ALT modifiers

Remove check for MOD_MASK_META as it is for <T- which never appears in TUI.
Make small changes to docs.

runtime/doc/cmdline.txt

@@ -240,7 +240,7 @@ CTRL-[						*c_CTRL-[* *c_<Esc>* *c_Esc*
 		Note: If your <Esc> key is hard to hit on your keyboard, train
 		yourself to use CTRL-[.
 						*c_META* *c_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have a
 		command-line mode mapping.
 							*c_CTRL-C*

runtime/doc/insert.txt

@@ -39,7 +39,7 @@ char		action	~
 		abbreviation.
 		Note: If your <Esc> key is hard to hit, try CTRL-[ instead.
 						*i_META* *i_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have an
 		insert-mode mapping.
 						*i_CTRL-C*

runtime/doc/intro.txt

@@ -383,8 +383,8 @@ Note:
   <k1>, ..., <k9> and <kPoint> will not work.
 - Nvim supports mapping multibyte chars with modifiers such as `<M-ä>`. Which
   combinations actually work depends on the the UI or host terminal.
-- When a key is pressed using a meta or alt modifier and no mapping exists
+- When a key is pressed using a meta or alt modifier and no mapping exists for
-  for that keypress, Nvim behaves as though <Esc> was pressed before the key.
+  that keypress, Nvim may behave as though <Esc> was pressed before the key.
 - It is possible to notate combined modifiers (e.g. <C-A-T> for CTRL-ALT-T),
   but your terminal must encode the input for that to work. |tui-input|
 

runtime/doc/vim_diff.txt

@@ -250,7 +250,7 @@ Input/Mappings:
   <M-1>, <M-BS>, <M-Del>, <M-Ins>, <M-/>, <M-\>, <M-Space>, <M-Enter>, etc.
   Case-sensitive: <M-a> and <M-A> are two different keycodes.
 
-  ALT behaves like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
+  ALT may behave like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
 
 Normal commands:
   |gO| shows a filetype-defined "outline" of the current buffer.

runtime/doc/visual.txt

@@ -161,9 +161,10 @@ If you want to highlight exactly the same area as the last time, you can use
 							*v_<Esc>*
 <Esc>			In Visual mode: Stop Visual mode.
 						*v_META* *v_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have a
 		visual-mode mapping.
+
 							*v_CTRL-C*
 CTRL-C			In Visual mode: Stop Visual mode.  When insert mode is
 			pending (the mode message shows

src/nvim/getchar.c

@@ -30,6 +30,7 @@
 #include "nvim/memline.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
+#include "nvim/mouse.h"
 #include "nvim/move.h"
 #include "nvim/normal.h"
 #include "nvim/ops.h"
@@ -1584,16 +1585,18 @@ int vgetc(void)
         vgetc_char = c;
       }
 
-      // If mappings are enabled (i.e., not Ctrl-v) and the user directly typed
+      // If mappings are enabled (i.e., not i_CTRL-V) and the user directly typed something with
-      // something with a meta- or alt- modifier that was not mapped, interpret
+      // MOD_MASK_ALT (<M-/<A- modifier) that was not mapped, interpret <M-x> as <Esc>x rather
-      // <M-x> as <Esc>x rather than as an unbound meta keypress. #8213
+      // than as an unbound <M-x> keypress. #8213
-      // In Terminal mode, however, this is not desirable. #16220
+      // In Terminal mode, however, this is not desirable. #16202 #16220
-      if (!no_mapping && KeyTyped && !(State & MODE_TERMINAL)
+      // Also do not do this for mouse keys, as terminals encode mouse events as CSI sequences, and
-          && (mod_mask == MOD_MASK_ALT || mod_mask == MOD_MASK_META)) {
+      // MOD_MASK_ALT has a meaning even for unmapped mouse keys.
+      if (!no_mapping && KeyTyped && mod_mask == MOD_MASK_ALT && !(State & MODE_TERMINAL)
+          && !is_mouse_key(c)) {
         mod_mask = 0;
         int len = ins_char_typebuf(c, 0);
         (void)ins_char_typebuf(ESC, 0);
-        ungetchars(len + 3);  // The ALT/META modifier takes three more bytes
+        ungetchars(len + 3);  // K_SPECIAL KS_MODIFIER MOD_MASK_ALT takes 3 more bytes
         continue;
       }
 

test/functional/editor/meta_key_spec.lua

@@ -91,7 +91,7 @@ describe('meta-keys #8226 #13042', function()
     command('tnoremap <A-j> alt-j')
     feed('i<M-l> xxx <A-j>')
     eq('meta-l xxx alt-j', exec_lua([[return _G.input_data]]))
-    -- Unmapped ALT-chord is sent to terminal as-is. #16220
+    -- Unmapped ALT-chord is sent to terminal as-is. #16202 #16220
     exec_lua([[_G.input_data = '']])
     command('tunmap <M-l>')
     feed('<M-l>')

test/functional/ui/statusline_spec.lua

@@ -21,11 +21,9 @@ describe('statusline clicks', function()
     command('set laststatus=2 mousemodel=extend')
     exec([=[
       function! MyClickFunc(minwid, clicks, button, mods)
-        let mods = trim(a:mods)
-        if mods ==# ''
         let g:testvar = printf("%d %d %s", a:minwid, a:clicks, a:button)
-        else
+        if a:mods !=# '    '
-          let g:testvar = printf("%d %d %s %s", a:minwid, a:clicks, a:button, mods)
+          let g:testvar ..= '(' .. a:mods .. ')'
         endif
       endfunction
     ]=])
@@ -35,8 +33,20 @@ describe('statusline clicks', function()
     meths.set_option('statusline', 'Not clicky stuff %0@MyClickFunc@Clicky stuff%T')
     meths.input_mouse('left', 'press', '', 0, 6, 17)
     eq('0 1 l', eval("g:testvar"))
+    meths.input_mouse('left', 'press', '', 0, 6, 17)
+    eq('0 2 l', eval("g:testvar"))
+    meths.input_mouse('left', 'press', '', 0, 6, 17)
+    eq('0 3 l', eval("g:testvar"))
+    meths.input_mouse('left', 'press', '', 0, 6, 17)
+    eq('0 4 l', eval("g:testvar"))
     meths.input_mouse('right', 'press', '', 0, 6, 17)
     eq('0 1 r', eval("g:testvar"))
+    meths.input_mouse('right', 'press', '', 0, 6, 17)
+    eq('0 2 r', eval("g:testvar"))
+    meths.input_mouse('right', 'press', '', 0, 6, 17)
+    eq('0 3 r', eval("g:testvar"))
+    meths.input_mouse('right', 'press', '', 0, 6, 17)
+    eq('0 4 r', eval("g:testvar"))
   end)
 
   it('works for winbar', function()
@@ -101,10 +111,30 @@ describe('statusline clicks', function()
 
   it("works with modifiers #18994", function()
     meths.set_option('statusline', 'Not clicky stuff %0@MyClickFunc@Clicky stuff%T')
-    meths.input_mouse('right', 'press', 's', 0, 6, 17)
+    -- Note: alternate between left and right mouse buttons to avoid triggering multiclicks
-    eq('0 1 r s', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'S', 0, 6, 17)
-    meths.input_mouse('left', 'press', 's', 0, 6, 17)
+    eq('0 1 l(s   )', eval("g:testvar"))
-    eq('0 1 l s', eval("g:testvar"))
+    meths.input_mouse('right', 'press', 'S', 0, 6, 17)
+    eq('0 1 r(s   )', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'A', 0, 6, 17)
+    eq('0 1 l(  a )', eval("g:testvar"))
+    meths.input_mouse('right', 'press', 'A', 0, 6, 17)
+    eq('0 1 r(  a )', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'AS', 0, 6, 17)
+    eq('0 1 l(s a )', eval("g:testvar"))
+    meths.input_mouse('right', 'press', 'AS', 0, 6, 17)
+    eq('0 1 r(s a )', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'T', 0, 6, 17)
+    eq('0 1 l(   m)', eval("g:testvar"))
+    meths.input_mouse('right', 'press', 'T', 0, 6, 17)
+    eq('0 1 r(   m)', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'TS', 0, 6, 17)
+    eq('0 1 l(s  m)', eval("g:testvar"))
+    meths.input_mouse('right', 'press', 'TS', 0, 6, 17)
+    eq('0 1 r(s  m)', eval("g:testvar"))
+    meths.input_mouse('left', 'press', 'C', 0, 6, 17)
+    eq('0 1 l( c  )', eval("g:testvar"))
+    -- <C-RightMouse> is for tag jump
   end)
 
   it("works for global statusline with vertical splits #19186", function()