tui: dump termcap info if -V3 ('verbose' >= 3)

Get terminal debugging info by starting Nvim with 'verbose' level 3:

    nvim -V3log

This is like Vim's `:set termcap`, which was removed in Nvim (and would
be very awkward to restore because of the decoupled UI).

runtime/doc/eval.txt

@@ -8889,11 +8889,6 @@ This does NOT work: >
 			value and the global value are changed.
 			Example: >
 				:let &path = &path . ',/usr/local/include'
-<			This also works for terminal codes in the form t_xx.
-			But only for alphanumerical names.  Example: >
-				:let &t_k1 = "\<Esc>[234;"
-<			When the code does not exist yet it will be created as
-			a terminal key code, there is no error.
 
 :let &{option-name} .= {expr1}
 			For a string option: Append {expr1} to the value.

runtime/doc/options.txt

@@ -6424,6 +6424,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Currently, these messages are given:
 	>= 1	When the shada file is read or written.
 	>= 2	When a file is ":source"'ed.
+	>= 3	UI info, terminal capabilities
 	>= 5	Every searched tags file and include file.
 	>= 8	Files for which a group of autocommands is executed.
 	>= 9	Every executed autocommand.

runtime/doc/starting.txt

@@ -249,14 +249,14 @@ argument.
 		for reading or writing a ShaDa file.  Can be used to find
 		out what is happening upon startup and exit.
 		Example: >
-			vim -V8 foobar
+			nvim -V8
 
 -V[N]{filename}
-		Like -V and set 'verbosefile' to {filename}.  The result is
-		that messages are not displayed but written to the file
-		{filename}.  {filename} must not start with a digit.
+		Like -V and set 'verbosefile' to {filename}.  Messages are not
+		displayed; instead they are written to the file {filename}.
+		{filename} must not start with a digit.
 		Example: >
-			vim -V20vimlog foobar
+			nvim -V20vimlog
 <
 							*-D*
 -D		Debugging.  Go to debugging mode when executing the first

runtime/doc/vim_diff.txt

@@ -327,22 +327,26 @@ Ed-compatible mode:
   ":set noedcompatible" is ignored
   ":set edcompatible" is an error
 
-	*t_xx* *:set-termcap* *termcap-options* *t_AB* *t_Sb* *t_vb* *t_SI*
+			*t_xx* *termcap-options* *t_AB* *t_Sb* *t_vb* *t_SI*
 Nvim does not have special `t_XX` options nor <t_XX> keycodes to configure
 terminal capabilities. Instead Nvim treats the terminal as any other UI. For
 example, 'guicursor' sets the terminal cursor style if possible.
 
-						  *'term'* *E529* *E530* *E531*
+			*:set-termcap*
+Start Nvim with 'verbose' level 3 to see the terminal capabilities. >
+	nvim -V3
+<
+			*'term'* *E529* *E530* *E531*
 'term' reflects the terminal type derived from |$TERM| and other environment
 checks.  For debugging only; not reliable during startup. >
 	:echo &term
 "builtin_x" means one of the |builtin-terms| was chosen, because the expected
 terminfo file was not found on the system.
 
-								*termcap*
+			*termcap*
 Nvim never uses the termcap database, only |terminfo| and |builtin-terms|.
 
-						*xterm-8bit* *xterm-8-bit*
+			*xterm-8bit* *xterm-8-bit*
 Xterm can be run in a mode where it uses true 8-bit CSI.  Supporting this
 requires autodetection of whether the terminal is in UTF-8 mode or non-UTF-8
 mode, as the 8-bit CSI character has to be written differently in each case.

src/nvim/README.md

@@ -32,6 +32,39 @@ The source files use extensions to hint about their purpose.
 - `*.h.generated.h` - exported functions’ declarations.
 - `*.c.generated.h` - static functions’ declarations.
 
+TUI debugging
+-------------
+
+### TUI troubleshoot
+
+Nvim logs its internal terminfo state at 'verbose' level 3.  This makes it
+possible to see exactly what terminfo values Nvim is using on any system.
+
+    nvim -V3log
+
+### TUI trace
+
+The ancient `script` command is still the "state of the art" for tracing
+terminal behavior. The libvterm `vterm-dump` utility formats the result for
+human-readability.
+
+Record a Nvim terminal session and format it with `vterm-dump`:
+
+    script foo
+    ./build/bin/nvim -u NONE
+    # Exit the script session with CTRL-d
+
+    # Use `vterm-dump` utility to format the result.
+    ./.deps/usr/bin/vterm-dump foo > bar
+
+Then you can compare `bar` with another session, to debug TUI behavior.
+
+### Terminal reference
+
+- `man terminfo`
+- http://bazaar.launchpad.net/~libvterm/libvterm/trunk/view/head:/doc/seqs.txt
+- http://invisible-island.net/xterm/ctlseqs/ctlseqs.html
+
 Nvim lifecycle
 --------------
 
@@ -39,7 +72,7 @@ Following describes how Nvim processes input.
 
 Consider a typical Vim-like editing session:
 
-01. Vim dispays the welcome screen
+01. Vim displays the welcome screen
 02. User types: `:`
 03. Vim enters command-line mode
 04. User types: `edit README.txt<CR>`

src/nvim/option.c

@@ -4906,15 +4906,14 @@ showoptions (
 
   vimoption_T **items = xmalloc(sizeof(vimoption_T *) * PARAM_COUNT);
 
-  /* Highlight title */
-  if (all == 2)
-    MSG_PUTS_TITLE(_("\n--- Terminal codes ---"));
-  else if (opt_flags & OPT_GLOBAL)
+  // Highlight title
+  if (opt_flags & OPT_GLOBAL) {
     MSG_PUTS_TITLE(_("\n--- Global option values ---"));
-  else if (opt_flags & OPT_LOCAL)
+  } else if (opt_flags & OPT_LOCAL) {
     MSG_PUTS_TITLE(_("\n--- Local option values ---"));
-  else
+  } else {
     MSG_PUTS_TITLE(_("\n--- Options ---"));
+  }
 
   /*
    * do the loop two times:

src/nvim/tui/terminfo.c

@@ -9,7 +9,10 @@
 #include <unibilium.h>
 
 #include "nvim/log.h"
+#include "nvim/globals.h"
 #include "nvim/memory.h"
+#include "nvim/message.h"
+#include "nvim/option.h"
 #include "nvim/tui/terminfo.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
@@ -166,3 +169,87 @@ unibi_term *terminfo_from_builtin(const char *term, char **termname)
   unibi_set_bool(ut, unibi_back_color_erase, false);
   return ut;
 }
+
+/// Dumps termcap info to the messages area.
+/// Serves a similar purpose as Vim `:set termcap` (removed in Nvim).
+///
+/// @note adapted from unibilium unibi-dump.c
+void terminfo_info_msg(const unibi_term *const ut)
+{
+  if (exiting) {
+    return;
+  }
+  msg_puts_title("\n\n--- Terminal info --- {{{\n");
+
+  char *term;
+  get_tty_option("term", &term);
+  msg_printf_attr(0, "&term: %s\n", term);
+  msg_printf_attr(0, "Description: %s\n", unibi_get_name(ut));
+  const char **a = unibi_get_aliases(ut);
+  if (*a) {
+    msg_puts("Aliases: ");
+    do {
+      msg_printf_attr(0, "%s%s\n", *a, a[1] ? " | " : "");
+      a++;
+    } while (*a);
+  }
+
+  msg_puts("Boolean capabilities:\n");
+  for (enum unibi_boolean i = unibi_boolean_begin_ + 1;
+       i < unibi_boolean_end_; i++) {
+    msg_printf_attr(0, "  %-25s %-10s = %s\n", unibi_name_bool(i),
+                    unibi_short_name_bool(i),
+                    unibi_get_bool(ut, i) ? "true" : "false");
+  }
+
+  msg_puts("Numeric capabilities:\n");
+  for (enum unibi_numeric i = unibi_numeric_begin_ + 1;
+       i < unibi_numeric_end_; i++) {
+    int n = unibi_get_num(ut, i);  // -1 means "empty"
+    msg_printf_attr(0, "  %-25s %-10s = %hd\n", unibi_name_num(i),
+                    unibi_short_name_num(i), n);
+  }
+
+  msg_puts("String capabilities:\n");
+  for (enum unibi_string i = unibi_string_begin_ + 1;
+       i < unibi_string_end_; i++) {
+    const char *s = unibi_get_str(ut, i);
+    if (s) {
+      msg_printf_attr(0, "  %-25s %-10s = ", unibi_name_str(i),
+                      unibi_short_name_str(i));
+      // Most of these strings will contain escape sequences.
+      msg_outtrans_special((char_u *)s, false);
+      msg_putchar('\n');
+    }
+  }
+
+  if (unibi_count_ext_bool(ut)) {
+    msg_puts("Extended boolean capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_bool(ut); i++) {
+      msg_printf_attr(0, "  %-25s = %s\n",
+                      unibi_get_ext_bool_name(ut, i),
+                      unibi_get_ext_bool(ut, i) ? "true" : "false");
+    }
+  }
+
+  if (unibi_count_ext_num(ut)) {
+    msg_puts("Extended numeric capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_num(ut); i++) {
+      msg_printf_attr(0, "  %-25s = %hd\n",
+                      unibi_get_ext_num_name(ut, i),
+                      unibi_get_ext_num(ut, i));
+    }
+  }
+
+  if (unibi_count_ext_str(ut)) {
+    msg_puts("Extended string capabilities:\n");
+    for (size_t i = 0; i < unibi_count_ext_str(ut); i++) {
+      msg_printf_attr(0, "  %-25s = ", unibi_get_ext_str_name(ut, i));
+      msg_outtrans_special((char_u *)unibi_get_ext_str(ut, i), false);
+      msg_putchar('\n');
+    }
+  }
+
+  msg_puts("}}}\n");
+  xfree(term);
+}

src/nvim/tui/tui.c

@@ -354,10 +354,12 @@ static void tui_main(UIBridgeData *bridge, UI *ui)
   tui_terminal_start(ui);
   data->stop = false;
 
-  // allow the main thread to continue, we are ready to start handling UI
-  // callbacks
+  // Allow main thread to continue, we are ready to handle UI callbacks.
   CONTINUE(bridge);
 
+  loop_schedule_deferred(&main_loop,
+                         event_create(show_termcap_event, 1, data->ut));
+
   while (!data->stop) {
     loop_poll_events(&tui_loop, -1);  // tui_loop.events is never processed
   }
@@ -1061,6 +1063,21 @@ static void tui_flush(UI *ui)
   flush_buf(ui, true);
 }
 
+/// Dumps termcap info to the messages area, if 'verbose' >= 3.
+static void show_termcap_event(void **argv)
+{
+  if (p_verbose < 3) {
+    return;
+  }
+  const unibi_term *const ut = argv[0];
+  if (!ut) {
+    abort();
+  }
+  // XXX: (future) if unibi_term is modified (e.g. after a terminal
+  // query-response) this is a race condition.
+  terminfo_info_msg(ut);
+}
+
 #ifdef UNIX
 static void suspend_event(void **argv)
 {

src/nvim/ui_bridge.c

@@ -82,6 +82,7 @@ UI *ui_bridge_attach(UI *ui, ui_main_fn ui_main, event_scheduler scheduler)
     abort();
   }
 
+  // Suspend the main thread until CONTINUE is called by the UI thread.
   while (!rv->ready) {
     uv_cond_wait(&rv->cond, &rv->mutex);
   }
@@ -149,7 +150,7 @@ static void ui_bridge_suspend(UI *b)
   uv_mutex_lock(&data->mutex);
   UI_BRIDGE_CALL(b, suspend, 1, b);
   data->ready = false;
-  // suspend the main thread until CONTINUE is called by the UI thread
+  // Suspend the main thread until CONTINUE is called by the UI thread.
   while (!data->ready) {
     uv_cond_wait(&data->cond, &data->mutex);
   }