NCursesUI: Add support for shifted function keys

Shifted function keys are not well standardized around terminals,
Shift F(N) usually returns F(X) + N, with X=12 on xterm, X=10 on
rxvt-unicode... Default to X=12 and make it configuable through
the ncurses_shift_function_key ui_option.

This fixes what #1898 tried to.

doc/pages/options.asciidoc

@@ -278,3 +278,7 @@ are exclusively available to built-in options.
 
         *ncurses_wheel_down_button*, *ncurses_wheel_up_button*:::
             specify which button send for wheel down/up events
+
+        *ncurses_shift_function_key*:::
+            Function key from which shifted function key start, if th
+            terminal sends F13 for <s-F1>, this should be set to 12.

src/main.cc

@@ -360,7 +360,8 @@ void register_options()
                        "    ncurses_enable_mouse          bool\n"
                        "    ncurses_change_colors         bool\n"
                        "    ncurses_wheel_up_button       int\n"
-                       "    ncurses_wheel_down_button     int\n",
+                       "    ncurses_wheel_down_button     int\n"
+                       "    ncurses_shift_function_key    int\n",
                        UserInterface::Options{});
     reg.declare_option("modelinefmt", "format string used to generate the modeline",
                        "%val{bufname} %val{cursor_line}:%val{cursor_char_column} {{context_info}} {{mode_info}} - %val{client}@[%val{session}]"_str);

src/ncurses_ui.cc

@@ -29,6 +29,8 @@ using std::max;
 
 struct NCursesWin : WINDOW {};
 
+constexpr int NCursesUI::default_shift_function_key;
+
 static constexpr StringView assistant_cat[] =
     { R"(  ___            )",
       R"( (__ \           )",
@@ -615,8 +617,10 @@ Optional<Key> NCursesUI::get_next_key()
 
         for (int i = 0; i < 12; ++i)
         {
-            if (c == KEY_F(i+1))
+            if (c == KEY_F(i + 1))
                 return {Key::F1 + i};
+            if (c == KEY_F(m_shift_function_key + i + 1))
+                return shift(Key::F1 + i);
         }
 
         if (c >= 0 and c < 256)
@@ -1082,6 +1086,13 @@ void NCursesUI::set_ui_options(const Options& options)
             (it->value == "yes" or it->value == "true");
     }
 
+    {
+        auto it = options.find("ncurses_shift_function_key"_sv);
+        m_shift_function_key = it != options.end() ?
+            str_to_int_ifp(it->value).value_or(default_shift_function_key)
+          : default_shift_function_key;
+    }
+
     {
         auto it = options.find("ncurses_change_colors"_sv);
         auto value = it == options.end() or

src/ncurses_ui.hh

@@ -140,6 +140,9 @@ private:
     int m_wheel_up_button = 4;
     int m_wheel_down_button = 5;
 
+    static constexpr int default_shift_function_key = 12;
+    int m_shift_function_key = default_shift_function_key;
+
     bool m_set_title = true;
     bool m_change_colors = true;