update PROMPT_SP heuristic

Update our implementation of the PROMPT_SP heuristic to match current
zsh behavior. This makes it behave better on terminals like ConEmu and
the native MS Windows console which automatically insert a newline when
writing to the last column of the line.

Fixes #789

src/env.cpp

@@ -14,6 +14,13 @@
 #include <time.h>
 #include <unistd.h>
 #include <wchar.h>
+
+#if HAVE_TERM_H
+#include <term.h>
+#elif HAVE_NCURSES_TERM_H
+#include <ncurses/term.h>
+#endif
+
 #include <algorithm>
 #include <map>
 #include <set>
@@ -50,6 +57,9 @@ extern char **environ;
 
 bool g_use_posix_spawn = false;  // will usually be set to true
 
+/// Does the terminal have the "eat_newline_glitch".
+bool term_has_xn = false;
+
 /// Struct representing one level in the function variable stack.
 struct env_node_t {
     /// Variable table.
@@ -245,6 +255,7 @@ static void handle_curses(const wchar_t *env_var_name) {
     // changed. At the present time it can be called just once. Also, we should really only do this
     // if the TERM var is set.
     // input_init();
+    term_has_xn = tgetflag((char *)"xn") == 1;  // does terminal have the eat_newline_glitch
 }
 
 /// React to modifying the given variable.

src/env.h

@@ -184,4 +184,6 @@ struct var_entry_t {
 };
 
 typedef std::map<wcstring, var_entry_t> var_table_t;
+
+extern bool term_has_xn;  // does the terminal have the "eat_newline_glitch"
 #endif

src/screen.cpp

@@ -31,6 +31,7 @@
 #include <vector>
 
 #include "common.h"
+#include "env.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "highlight.h"
 #include "output.h"
@@ -1194,7 +1195,8 @@ void s_reset(screen_t *s, screen_reset_mode_t mode) {
         // Don't need to check for fish_wcwidth errors; this is done when setting up
         // omitted_newline_char in common.cpp.
         int non_space_width = fish_wcwidth(omitted_newline_char);
-        if (screen_width >= non_space_width) {
+        // We do `>` rather than `>=` because the code below might require one extra space.
+        if (screen_width > non_space_width) {
             bool justgrey = true;
             if (enter_dim_mode) {
                 std::string dim = tparm(enter_dim_mode);
@@ -1224,16 +1226,22 @@ void s_reset(screen_t *s, screen_reset_mode_t mode) {
                 abandon_line_string.append(
                     str2wcstring(tparm(exit_attribute_mode)));  // normal text ANSI escape sequence
             }
-            abandon_line_string.append(screen_width - non_space_width, L' ');
+            int newline_glitch_width = term_has_xn ? 0 : 1;
+            abandon_line_string.append(screen_width - non_space_width - newline_glitch_width, L' ');
         }
         abandon_line_string.push_back(L'\r');
+        abandon_line_string.push_back(omitted_newline_char);
         // Now we are certainly on a new line. But we may have dropped the omitted newline char on
         // it. So append enough spaces to overwrite the omitted newline char, and then clear all the
-        // spaces from the new line
+        // spaces from the new line.
         abandon_line_string.append(non_space_width, L' ');
         abandon_line_string.push_back(L'\r');
-        // clear entire line - el2
-        abandon_line_string.append(L"\x1b[2K");
+        // Clear entire line. Zsh doesn't do this. Fish added this with commit 4417a6ee: If you have
+        // a prompt preceded by a new line, you'll get a line full of spaces instead of an empty
+        // line above your prompt. This doesn't make a difference in normal usage, but copying and
+        // pasting your terminal log becomes a pain. This commit clears that line, making it an
+        // actual empty line.
+        abandon_line_string.append(L"\e[2K");
 
         const std::string narrow_abandon_line_string = wcs2string(abandon_line_string);
         write_loop(STDOUT_FILENO, narrow_abandon_line_string.c_str(),