Restore diff based terminal output optimization when synchronized

terminal_synchronized ui_option now also controls this behaviour,
update out of date documentation for ui_options as well.

As discussed in #4317

doc/pages/options.asciidoc

@@ -349,10 +349,6 @@ are exclusively available to built-in options.
         *terminal_enable_mouse*:::
             boolean option that enables mouse support
 
-        *terminal_change_colors*:::
-            boolean option that can disable color palette changing if the
-            terminfo enables it but the terminal does not support it.
-
         *terminal_shift_function_key*:::
             Function key from which shifted function key start, if the
             terminal sends F13 for <s-F1>, this should be set to 12.
@@ -367,8 +363,9 @@ are exclusively available to built-in options.
             padding line (defaults to *false*)
 
         *terminal_synchronized*:::
-            if *yes* or *true*, emit iterm2 synchronized output escape sequences
+            if *yes* or *true*, emit synchronized output escape sequences and
-            during redraw to reduce flickering (defaults to *false*)
+            reduce terminal output with sequences that could trigger flickering
+            if unsynchronized (defaults to *false*)
 
 [[startup-info]]
 *startup_info_version* `int`::

src/main.cc

@@ -545,9 +545,11 @@ void register_options()
                        "    terminal_status_on_top         bool\n"
                        "    terminal_set_title             bool\n"
                        "    terminal_enable_mouse          bool\n"
-                       "    terminal_change_colors         bool\n"
+                       "    terminal_synchronized          bool\n"
                        "    terminal_wheel_scroll_amount   int\n"
-                       "    terminal_shift_function_key    int\n",
+                       "    terminal_shift_function_key    int\n"
+                       "    terminal_padding_char          codepoint\n"
+                       "    terminal_padding_fill          bool\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/terminal_ui.cc

@@ -284,10 +284,6 @@ void TerminalUI::Screen::output(bool force, bool synchronized, Writer& writer)
     if (not lines)
         return;
 
-    // iTerm2 "begin synchronized update" sequence
-    if (synchronized)
-        writer.write("\033[?2026h");
-
     if (force)
     {
         std::fill_n(hashes.get(), (size_t)size.line, 0);
@@ -299,17 +295,9 @@ void TerminalUI::Screen::output(bool force, bool synchronized, Writer& writer)
         return (hash_value(line.atoms) << 1) | 1; // ensure non-zero
     };
 
-    for (int line = 0; line < (int)size.line; ++line)
+    auto output_line = [&](const Line& line) {
-    {
-        auto hash = hash_line(lines[line]); 
-        if (hash == hashes[line])
-            continue;
-        hashes[line] = hash;
-
-        format_with(writer, "\033[{}H", line + 1);
-
         ColumnCount pending_move = 0;
-        for (auto& [text, skip, face] : lines[line].atoms)
+        for (auto& [text, skip, face] : line.atoms)
         {
             if (text.empty() and skip == 0)
                 continue;
@@ -329,11 +317,75 @@ void TerminalUI::Screen::output(bool force, bool synchronized, Writer& writer)
             else if (skip > 0)
                 writer.write(String{' ', skip});
         }
-    }
+    };
 
-    // iTerm2 "end synchronized update" sequence
     if (synchronized)
-        writer.write("\033[?2026l");
+    {
+        writer.write("\033[?2026h"); // begin synchronized update
+
+        struct Change { int keep; int add; int del; };
+        Vector<Change> changes{Change{}};
+        auto new_hashes = ArrayView{lines.get(), (size_t)size.line} | transform(hash_line);
+        for_each_diff(hashes.get(), (int)size.line,
+                      new_hashes.begin(), (int)size.line,
+                      [&changes](DiffOp op, int len) mutable {
+            switch (op)
+            {
+                case DiffOp::Keep:
+                    changes.push_back({len, 0, 0});
+                    break;
+                case DiffOp::Add:
+                    changes.back().add += len;
+                    break;
+                case DiffOp::Remove:
+                    changes.back().del += len;
+                    break;
+            }
+        });
+        std::copy(new_hashes.begin(), new_hashes.end(), hashes.get());
+
+        int line = 0;
+        for (auto& change : changes)
+        {
+            line += change.keep;
+            if (int del = change.del - change.add; del > 0)
+            {
+                format_with(writer, "\033[{}H\033[{}M", line + 1, del);
+                line -= del;
+            }
+            line += change.del;
+        }
+
+        line = 0;
+        for (auto& change : changes)
+        {
+            line += change.keep;
+            for (int i = 0; i < change.add; ++i)
+            {
+                if (int add = change.add - change.del; i == 0 and add > 0)
+                    format_with(writer, "\033[{}H\033[{}L", line + 1, add);
+                else
+                    format_with(writer, "\033[{}H", line + 1);
+
+                output_line(lines[line++]);
+            }
+        }
+
+        writer.write("\033[?2026l"); // end synchronized update
+    }
+    else
+    {
+        for (int line = 0; line < (int)size.line; ++line)
+        {
+            auto hash = hash_line(lines[line]); 
+            if (hash == hashes[line])
+                continue;
+            hashes[line] = hash;
+
+            format_with(writer, "\033[{}H", line + 1);
+            output_line(lines[line]);
+        }
+    }
 }
 
 constexpr int TerminalUI::default_shift_function_key;