Treat unmapped ALT/META as ESC+c in all modes

In #8226 <A-x> and <M-x> were changed to behave like <Esc>x in insert
mode when no mapping exists. This commit backs out that change and
replaces it with a more general one that makes unmapped ALT and META
keypresses as <Esc>+char in all modes. This fixes an unnecessary and
confusing inconsistency between modes.

5 files changed, 13 insertions, 4 deletions

diff --git a/runtime/doc/cmdline.txt b/runtime/doc/cmdline.txt
index 163dc81804..f7a281cb88 100644
--- a/runtime/doc/cmdline.txt
+++ b/runtime/doc/cmdline.txt
@@ -224,6 +224,10 @@ CTRL-[						*c_CTRL-[* *c_<Esc>* *c_Esc*
 		present in 'cpoptions', start entered command.
 		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.
+		For example <A-x> acts like <Esc>x if <A-x> does not have a
+		command-line mode mapping.
 							*c_CTRL-C*
 CTRL-C		quit command-line without executing
 
diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index e53af5074b..c4b93a2a27 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -42,9 +42,9 @@ 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|) acts 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.
+		insert-mode mapping.
 						*i_CTRL-C*
 CTRL-C		Quit insert mode, go back to Normal mode.  Do not check for
 		abbreviations.  Does not trigger the |InsertLeave| autocommand
diff --git a/runtime/doc/intro.txt b/runtime/doc/intro.txt
index 59b1f44e39..d858985e3f 100644
--- a/runtime/doc/intro.txt
+++ b/runtime/doc/intro.txt
@@ -382,6 +382,8 @@ 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
+  for that keypress, Nvim behaves as though <Esc> was pressed before the key.
 
 								*<>*
 Examples are often given in the <> notation.  Sometimes this is just to make
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 3090be82f2..ae60c1c5e8 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -195,7 +195,7 @@ 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 in insert-mode behaves like <Esc> if not mapped. |i_ALT|
+  ALT behaves like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
 
 Normal commands:
   |g<Tab>| goes to the last-accessed tabpage.
diff --git a/runtime/doc/visual.txt b/runtime/doc/visual.txt
index 0052382044..fd3d93ed98 100644
--- a/runtime/doc/visual.txt
+++ b/runtime/doc/visual.txt
@@ -159,7 +159,10 @@ 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.
+		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