perf: fast display (row runs, bulk fills, pixel scroll), terminal draws changed cells only + scrolls pixels, print! batched (was IPC per fmt fragment), snake sleeps on timer

crates/servers/terminal-server/src/tests/server.rs

@@ -16,15 +16,23 @@ struct Fake {
     flushes: usize,
     timers: usize,
     focus_taken: usize,
+    scrolled: usize,
+    /// Every row drawn, with how many cells.
+    rows_drawn: Vec<(usize, usize)>,
     interrupted: Vec<CapSlot>,
 }
 
 impl Console for Fake {
-    fn draw_row(&mut self, _row: usize, _text: &[u8]) {}
+    fn draw_row(&mut self, row: usize, text: &[u8]) {
+        self.rows_drawn.push((row, text.len()));
+    }
     fn draw_cursor(&mut self, _col: usize, _row: usize, _under: u8) {}
     fn flush(&mut self) {
         self.flushes += 1;
     }
+    fn scroll(&mut self, rows: usize) {
+        self.scrolled += rows;
+    }
     fn flush_later(&mut self) {
         self.timers += 1;
     }
@@ -143,3 +151,28 @@ fn redraw_takes_focus_back_and_paints_every_row() {
     assert_eq!(t.console().focus_taken, 1);
     assert_eq!(&t.grid().row(0)[..2], b"hi");
 }
+
+#[test]
+fn a_full_screen_scrolls_its_pixels_and_draws_one_row() {
+    let mut t = term(&[]);
+    for _ in 0..12 {
+        t.handle_call(1, text_words(b"line\n"), None);
+    }
+    // 10 rows: two scrolls so far, then one more line.
+    t.console_mut_for_test().rows_drawn.clear();
+    let before = t.console().scrolled;
+    t.handle_call(1, text_words(b"more\n"), None);
+    assert_eq!(t.console().scrolled, before + 1, "the screen moved up a row");
+    let drawn = &t.console().rows_drawn;
+    assert!(drawn.len() <= 2, "only the rows that changed: {drawn:?}");
+}
+
+#[test]
+fn rows_are_drawn_only_as_wide_as_their_text_or_what_it_replaces() {
+    let mut t = term(&[]);
+    t.handle_call(1, text_words(b"hello"), None);
+    assert_eq!(t.console().rows_drawn.last(), Some(&(0, 5)));
+    t.handle_call(1, [types::terminal::CLEAR, 0, 0, 0], None);
+    // Cleared: the old five cells and the cursor after them are erased.
+    assert!(t.console().rows_drawn.contains(&(0, 6)), "{:?}", t.console().rows_drawn);
+}