term.ui: feature-detect the SU spec (#6844)

vlib/term/ui/input.v

@@ -168,6 +168,7 @@ mut:
 	read_buf      []byte
 	print_buf     []byte
 	paused        bool
+	enable_su     bool
 pub mut:
 	frame_count   u64
 	window_width  int

vlib/term/ui/termios_nix.c.v

@@ -75,13 +75,31 @@ fn (mut ctx Context) termios_setup() {
 		print('\x1b]0;$ctx.cfg.window_title\x07')
 	}
 
+	C.tcsetattr(C.STDIN_FILENO, C.TCSAFLUSH, &termios)
+	// feature-test the SU spec
+	sx, sy := get_cursor_position()
+	print('$bsu$esu')
+	ex, ey := get_cursor_position()
+
+	if sx == ex && sy == ey {
+		// the terminal either ignored or handled the sequence properly, enable SU
+		ctx.enable_su = true
+	} else {
+		ctx.draw_line(sx, sy, ex, ey)
+		ctx.set_cursor_position(sx, sy)
+		ctx.flush()
+	}
 	// Prevent stdin from blocking by making its read time 0
 	termios.c_cc[C.VTIME] = 0
 	termios.c_cc[C.VMIN] = 0
 	C.tcsetattr(C.STDIN_FILENO, C.TCSAFLUSH, &termios)
+	// enable mouse input
 	print('\x1b[?1003h\x1b[?1006h')
 	if ctx.cfg.use_alternate_buffer {
+		// switch to the alternate buffer
 		print('\x1b[?1049h')
+		// clear the terminal and set the cursor to the origin
+		print('\x1b[2J\x1b[3J\x1b[1;1H')
 	}
 	ctx.termios = termios
 	ctx.window_height, ctx.window_width = get_terminal_size()
@@ -126,9 +144,41 @@ fn (mut ctx Context) termios_setup() {
 			c.event(event)
 		}
 	})
+
 	os.flush()
 }
 
+fn get_cursor_position() (int, int) {
+	print('\033[6n')
+	// ESC [ YYY `;` XXX `R`
+	mut reading_x, mut reading_y := false, false
+	mut x, mut y := 0, 0
+	for i := 0; i < 15 ; i++ {
+		ch := int(C.getchar())
+		b := byte(ch)
+		i++
+		// state management:
+		if b == `R` {
+			break
+		} else if b == `[` {
+			reading_y = true
+			reading_x = false
+			continue
+		} else if b == `;` {
+			reading_y = false
+			reading_x = true
+			continue
+		}
+		// converting string vals to ints:
+		if reading_x {
+			x = x * 10 + b - byte(`0`)
+		} else if reading_y {
+			y = y * 10 + b - byte(`0`)
+		}
+	}
+	return x, y
+}
+
 fn termios_reset() {
 	C.tcsetattr(C.STDIN_FILENO, C.TCSAFLUSH /* C.TCSANOW ?? */, &termios_at_startup)
 	print('\x1b[?1003l\x1b[?1006l\x1b[?25h')

vlib/term/ui/ui.v

@@ -1,6 +1,5 @@
 module ui
 
-import os
 import strings
 
 pub struct Color {
@@ -19,7 +18,6 @@ pub fn (c Color) hex() string {
 const (
 	bsu = '\x1bP=1s\x1b\\'
 	esu = '\x1bP=2s\x1b\\'
-	vno_bsu_esu = os.getenv('VNO_BSU_ESU').len>0
 )
 
 [inline]
@@ -34,7 +32,7 @@ pub fn (mut ctx Context) flush() {
 		// TODO
 	} $else {
 		// TODO: Diff the previous frame against this one, and only render things that changed?
-		if vno_bsu_esu {
+		if !ctx.enable_su {
 			C.write(C.STDOUT_FILENO, ctx.print_buf.data, ctx.print_buf.len)
 		} else {
 			C.write(C.STDOUT_FILENO, bsu.str, bsu.len)