Add 'terminal_synchronized' ui_option to opt-in synchronized output

Synchronized output does not work well with various terminals
(including the linux console). It should also be unnecessary when
not going through a slow link.

This will eventually be removed if it is not proven to be useful
to some users.

doc/pages/options.asciidoc

@@ -360,15 +360,19 @@ are exclusively available to built-in options.
             Function key from which shifted function key start, if the
             terminal sends F13 for <s-F1>, this should be set to 12.
 
-        *ncurses_padding_char*:::
+        *terminal_padding_char*:::
             character used to indicate the area out of the displayed buffer
             (defaults to '~')
 
-        *ncurses_padding_fill*:::
+        *terminal_padding_fill*:::
             if *yes* or *true*, fill the padding area with the padding character
             instead of displaying a single character at the beginning of the
             padding line (defaults to *false*)
 
+        *terminal_synchronize*:::
+            if *yes* or *true*, emit iterm2 synchronized output escape sequences
+            during redraw to reduce flickering (defaults to *false*)
+
 [[startup-info]]
 *startup_info_version* `int`::
     _default_ 0 +

src/terminal_ui.cc

@@ -221,13 +221,14 @@ void TerminalUI::Screen::set_face(const Face& face)
     m_active_face = face;
 }
 
-void TerminalUI::Screen::output(bool force)
+void TerminalUI::Screen::output(bool force, bool synchronized)
 {
     if (lines.empty())
         return;
 
-    // iTerm2 "begin synchronised update" sequence
-    printf("\033P=1s\033\\");
+    // iTerm2 "begin synchronized update" sequence
+    if (synchronized)
+        printf("\033P=1s\033\\");
 
     if (force)
     {
@@ -304,8 +305,9 @@ void TerminalUI::Screen::output(bool force)
         }
     }
 
-    // iTerm2 "endsynchronised update" sequence
-    printf("\033P=2s\033\\");
+    // iTerm2 "end synchronized update" sequence
+    if (synchronized)
+        printf("\033P=2s\033\\");
 }
 
 constexpr int TerminalUI::default_shift_function_key;
@@ -450,7 +452,7 @@ void TerminalUI::redraw(bool force)
 
     m_info.blit(m_screen);
 
-    m_screen.output(force);
+    m_screen.output(force, m_synchronized);
 
     auto set_cursor_pos = [](DisplayCoord c) {
         printf("\033[%d;%dH", (int)c.line + 1, (int)c.column + 1);
@@ -1387,17 +1389,14 @@ void TerminalUI::set_ui_options(const Options& options)
             m_assistant = ConstArrayView<StringView>{};
     }
 
-    {
-        auto it = options.find("terminal_status_on_top"_sv);
-        m_status_on_top = it != options.end() and
-            (it->value == "yes" or it->value == "true");
-    }
+    auto get_boolean_option = [&](StringView name, bool default_value) {
+        auto it = options.find(name);
+        return it == options.end() ? default_value
+                                   : (it->value == "yes" or it->value == "true");
+    };
 
-    {
-        auto it = options.find("terminal_set_title"_sv);
-        m_set_title = it == options.end() or
-            (it->value == "yes" or it->value == "true");
-    }
+    m_status_on_top = get_boolean_option("terminal_status_on_top", false);
+    m_set_title = get_boolean_option("terminal_set_title", true);
 
     {
         auto it = options.find("terminal_shift_function_key"_sv);
@@ -1407,10 +1406,7 @@ void TerminalUI::set_ui_options(const Options& options)
     }
 
     {
-        auto enable_mouse_it = options.find("terminal_enable_mouse"_sv);
-        enable_mouse(enable_mouse_it == options.end() or
-                     enable_mouse_it->value == "yes" or
-                     enable_mouse_it->value == "true");
+        enable_mouse(get_boolean_option("terminal_enable_mouse", true));
 
         auto wheel_up_it = options.find("terminal_wheel_up_button"_sv);
         m_wheel_up_button = wheel_up_it != options.end() ?
@@ -1437,11 +1433,8 @@ void TerminalUI::set_ui_options(const Options& options)
             m_padding_char = it->value[0_char];
     }
 
-    {
-        auto it = options.find("terminal_padding_fill"_sv);
-        m_padding_fill = it != options.end() and
-            (it->value == "yes" or it->value == "true");
-    }
+    m_padding_fill = get_boolean_option("terminal_padding_fill", false);
+    m_synchronized = get_boolean_option("terminal_synchronized", false);
 }
 
 }

src/terminal_ui.hh

@@ -88,7 +88,7 @@ private:
 
     struct Screen : Window
     {
-        void output(bool force);
+        void output(bool force, bool synchronized);
         void set_face(const Face& face);
 
         Vector<size_t> hashes;
@@ -152,6 +152,7 @@ private:
     int m_shift_function_key = default_shift_function_key;
 
     bool m_set_title = true;
+    bool m_synchronized = false;
 
     Codepoint m_padding_char = '~';
     bool m_padding_fill = false;