Fix case-changing autosuggestions generated mid-token

This fixes a bug where a case-changing autosuggestion generated from the
middle of a token would append too much to the end of the token.

Fixes #8820

CHANGELOG.rst

@@ -33,6 +33,7 @@ Interactive improvements
 ------------------------
 - The default command-not-found handler now reports a special error if there is a non-executable file (:issue:`8804`)
 - ``less`` and other interactive commands would occasionally be stopped when run in a pipeline with fish functions; this has been fixed (:issue:`8699`).
+- Case-changing autosuggestions generated mid-token now correctly append only the suffix, instead of duplicating the token (:issue:`8820`).
 
 New or improved bindings
 ^^^^^^^^^^^^^^^^^^^^^^^^

src/fish_tests.cpp

@@ -3327,6 +3327,13 @@ static void test_complete() {
         completions.front().completion, completions.front().flags, cmdline, &where, false);
     do_test(newcmdline == L"touch test/complete_test/bracket\\[abc\\] ");
 
+    // #8820
+    size_t cursor_pos = 11;
+    newcmdline =
+        completion_apply_to_command_line(L"Debug/", COMPLETE_REPLACES_TOKEN | COMPLETE_NO_SPACE,
+                                         L"mv debug debug", &cursor_pos, true);
+    do_test(newcmdline == L"mv debug Debug/");
+
 #ifndef __CYGWIN__  // Square brackets are not legal path characters on WIN32/CYGWIN
     cmdline = LR"(touch test/complete_test/gnarlybracket\\[)";
     completions = do_complete(cmdline, {});

src/reader.cpp

@@ -1615,8 +1615,7 @@ wcstring completion_apply_to_command_line(const wcstring &val, complete_flags_t
         const wchar_t *begin, *end;
 
         const wchar_t *buff = command_line.c_str();
-        parse_util_token_extent(buff, cursor_pos, &begin, nullptr, nullptr, nullptr);
+        parse_util_token_extent(buff, cursor_pos, &begin, &end, nullptr, nullptr);
-        end = buff + cursor_pos;
 
         wcstring sb(buff, begin - buff);