Shell: Always print prompt to new line

This is only done on non banan-os targets if terminal is detected to
support cursor position querying

userspace/programs/Shell/main.cpp

@@ -11,6 +11,7 @@
 #include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/select.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <termios.h>
@@ -1243,6 +1244,101 @@ static void print_prompt()
 	fflush(stdout);
 }
 
+static bool detect_cursor_position_support()
+{
+	constexpr auto getchar_nonblock =
+		[]() -> char
+		{
+			fd_set fds;
+			FD_ZERO(&fds);
+			FD_SET(STDIN_FILENO, &fds);
+
+			timeval timeout;
+			timeout.tv_sec = 0;
+			timeout.tv_usec = 100'000;
+
+			int nselect = select(STDIN_FILENO + 1, &fds, nullptr, nullptr, &timeout);
+			if (nselect != 1)
+				return '\0';
+
+			char ch;
+			if (read(STDIN_FILENO, &ch, 1) != 1)
+				return '\0';
+			return ch;
+		};
+
+	if (write(STDOUT_FILENO, "\e[6n", 4) != 4)
+		return false;
+
+	char ch = getchar_nonblock();
+	if (ch != '\e')
+	{
+		if (ch != '\0')
+			ungetc(ch, stdin);
+		return false;
+	}
+	if (getchar_nonblock() != '[')
+		return false;
+
+	int cur;
+	while (isdigit(cur = getchar_nonblock()))
+		;
+	if (cur != ';')
+		return false;
+	while (isdigit(cur = getchar_nonblock()))
+		;
+	if (cur != 'R')
+		return false;
+
+	return true;
+}
+
+struct CursorPosition
+{
+	int x;
+	int y;
+};
+
+static BAN::Optional<CursorPosition> try_read_cursor_position()
+{
+#if __banan_os__
+	return {};
+#endif
+
+	static BAN::Optional<bool> s_supports_cursor_position;
+	if (!s_supports_cursor_position.has_value())
+		s_supports_cursor_position = detect_cursor_position_support();
+
+	if (!s_supports_cursor_position.value())
+		return {};
+
+	if (write(STDOUT_FILENO, "\e[6n", 4) != 4)
+		return {};
+
+	char ch = getchar();
+	if (ch != '\e')
+	{
+		ungetc(ch, stdin);
+		return {};
+	}
+	if (getchar() != '[')
+		return {};
+
+	int cur, x = 0, y = 0;
+	while (isdigit(cur = getchar()))
+		y = (y * 10) + (cur - '0');
+	if (cur != ';')
+		return {};
+	while (isdigit(cur = getchar()))
+		x = (x * 10) + (cur - '0');
+	if (cur != 'R')
+		return {};
+
+	if (x > 0) x--;
+	if (y > 0) y--;
+	return CursorPosition { x, y };
+}
+
 int main(int argc, char** argv)
 {
 	realpath(argv[0], s_shell_path);
@@ -1466,6 +1562,10 @@ int main(int argc, char** argv)
 				MUST(history.push_back(buffers[index]));
 				buffers = history;
 				MUST(buffers.emplace_back(""_sv));
+
+				auto cursor_pos = try_read_cursor_position();
+				if (cursor_pos.has_value() && cursor_pos->x > 0)
+					printf("\e[7m%%\e[m\n");
 			}
 			print_prompt();
 			index = buffers.size() - 1;