patch 9.1.2048: MS-Windows: backspace behavior wrong with ConPTY

Problem:  Pressing backspace in ConPTY on Windows deletes an entire
          word, not just a single character. This is the same as
          pressing Alt + backspace in cmd.exe.
Solution: When using ConPTY on Windows, the `VTERM_KEY_BACKSPACE` key is
          now sent when the Backspace key or Ctrl+H is detected.
          (Muraoka Taro)

Historically, Windows has assigned the code 0x7F (DEL) to backspace, and
it seems necessary to follow that behavior when using ConPTY.

closes: #19065

Signed-off-by: Muraoka Taro <koron.kaoriya@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: koron

src/terminal.c

@@ -1584,9 +1584,26 @@ term_convert_key(term_T *term, int c, int modmask, char *buf)
     {
 	// don't use VTERM_KEY_ENTER, it may do an unwanted conversion
 
-				// don't use VTERM_KEY_BACKSPACE, it always
-				// becomes 0x7f DEL
-	case K_BS:		c = term_backspace_char; break;
+	case K_BS:
+#ifdef MSWIN
+	    // In ConPTY, we must use VTERM_KEY_BACKSPACE, otherwise it will
+	    // delete one word, equivalent to Alt+Backspace.
+	    if (term->tl_conpty)
+		key = VTERM_KEY_BACKSPACE;
+	    else
+#endif
+		// don't use VTERM_KEY_BACKSPACE, it always becomes 0x7f DEL
+		c = term_backspace_char;
+	    break;
+
+#ifdef MSWIN
+	case BS:
+	    // In ConPTY, we must use VTERM_KEY_BACKSPACE, otherwise it will
+	    // delete one word, equivalent to Alt+Backspace.
+	    if (term->tl_conpty)
+		key = VTERM_KEY_BACKSPACE;
+	    break;
+#endif
 
 	case ESC:		key = VTERM_KEY_ESCAPE; break;
 	case K_DEL:		key = VTERM_KEY_DEL; break;

src/testdir/test_terminal3.vim

@@ -1090,4 +1090,38 @@ func Test_terminal_ansi_color_windows_cui()
   call assert_equal(expected_colors, term_scrape(buf, 1)[:len_to_check-1]->map({_, v -> v['bg']}))
 endfunc
 
+func Test_terminal_backspace_on_windows()
+  if !has('win32')
+    throw 'Skipped: only for the Windows CUI'
+  endif
+  " Specify a simple prompt for easy comparison
+  let save_prompt = $PROMPT
+  let $PROMPT = '>'
+
+  " Return the prompt line before the cursor
+  func s:get_cmd_prompt(buf)
+    let cur = term_getcursor(a:buf)
+    return term_getline(a:buf, cur[0])[:cur[1]-2]
+  endfunc
+
+  let buf = term_start('cmd.exe')
+  call WaitForAssert({-> assert_equal('>', s:get_cmd_prompt(buf))}, 100)
+
+  " Verify sent characters are echoed back
+  call term_sendkeys(buf, 'foo bar')
+  call WaitForAssert({-> assert_equal('>foo bar', s:get_cmd_prompt(buf))}, 100)
+  " Backspace should delete a character in front of the cursor
+  call term_sendkeys(buf, "\<BS>")
+  call WaitForAssert({-> assert_equal('>foo ba', s:get_cmd_prompt(buf))}, 100)
+  " Ctrl+H behaves like Backspace
+  call term_sendkeys(buf, "\<C-H>")
+  call WaitForAssert({-> assert_equal('>foo b', s:get_cmd_prompt(buf))}, 100)
+  " Send a total of four BS and Ctrl+H to erase four characters.
+  call term_sendkeys(buf, "\<BS>\<BS>\<C-H>\<C-H>")
+  call WaitForAssert({-> assert_equal('>f', s:get_cmd_prompt(buf))}, 100)
+
+  delfunc s:get_cmd_prompt
+  let $PROMPT = save_prompt
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2048,
 /**/
     2047,
 /**/