vt: defer scrolls and batch framebuffer pushes

Streaming output scrolled one framebuffer push per line (each a
~3-8 MB write-only copy).  The vt now scrolls the logical grid
instantly in RAM and defers the display shift: N pending scrolls are
pushed together as one shift_up_n — one push regardless of batch size.

- vt: PENDING_SCROLLS counter; scroll_up shifts CELLS only; flush
  renders nothing while pending (the diff would otherwise re-rasterise
  the whole screen per line); sync_scroll() does one gop::shift_up_n +
  RENDERED mirror + normal diff flush (which re-blits just the newly
  exposed rows and the cursor).
- Sync triggers: batch of 16 rows, ~40 ms of stream pause (100 Hz
  TICKS), any CSI sequence (full-screen apps like the editor render
  immediately), clear_all/set_cursor_visible, interactive input
  (tty read_line/read_byte), and the shell before each prompt.
- gop: shift_up → shift_up_n(n) (one shadow memcpy + one push).

Verified in QEMU: mmap/hexdump streaming renders in coherent batches
with the prompt syncing at the end; clear wipes clean; the editor
scrolls and redraws immediately (45-line file, gutter + status bar
intact, cursor correct); save/exit/cat round-trip unchanged.  Release
and dev builds pass.

1 files changed, 66 insertions, 5 deletions

diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 4db08fc..88129f2 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -119,6 +119,7 @@ pub fn size() -> (usize, usize) {
 }
 
 pub fn set_cursor_visible(v: bool) {
+    sync_scroll();
     unsafe {
         CURSOR_VISIBLE = v;
     }
@@ -261,6 +262,12 @@ fn write_char(c: char) {
 }
 
 fn csi_final(b: u8) {
+    // A CSI sequence means a full-screen app is actively rendering —
+    // push any deferred scrolls so it draws over a current framebuffer
+    // (streaming shell output never emits CSI, so it stays batched).
+    if unsafe { PENDING_SCROLLS } > 0 {
+        sync_scroll();
+    }
     match b {
         // Cursor position (1-based; 0 means 1)
         b'H' | b'f' => {
@@ -400,18 +407,62 @@ fn newline() {
     }
 }
 
-/// Shift the whole screen up one row: cell grids in memory, pixels on the
-/// framebuffer.  Only the exposed bottom row gets re-rasterised by `flush`.
+/// Deferred scrolls: the logical grid scrolls instantly (RAM), but the
+/// framebuffer shift+push is batched.  N pending scrolls cost one push,
+/// so a burst of streaming output no longer writes the whole framebuffer
+/// once per line.  `flush` renders nothing while scrolls are pending;
+/// `sync_scroll` pushes the whole batch and lets the normal diff re-blit
+/// only the newly-exposed rows.
+const SCROLL_BATCH: usize = 16;
+static mut PENDING_SCROLLS: usize = 0;
+static mut LAST_SYNC_TICK: u16 = 0;
+
+/// Shift the logical grid up one row; defer the framebuffer shift.
 fn scroll_up() {
     let n = unsafe { ROWS * COLS };
     let cols = unsafe { COLS };
     cells().copy_within(cols..n, 0);
-    rendered_cells().copy_within(cols..n, 0);
     for c in 0..cols {
         cells()[n - cols + c] = BLANK;
-        rendered_cells()[n - cols + c] = BLANK;
     }
-    gop::shift_up();
+    unsafe {
+        PENDING_SCROLLS += 1;
+        // Batch: sync when enough scrolls piled up, or when the stream
+        // pauses (~40 ms at the 100 Hz timer) so interactive output still
+        // appears promptly.
+        const TICK_LIMIT: u16 = 4;
+        let now = crate::interrupts::TICKS.load(core::sync::atomic::Ordering::Relaxed);
+        if PENDING_SCROLLS >= SCROLL_BATCH
+            || now.wrapping_sub(LAST_SYNC_TICK) > TICK_LIMIT
+        {
+            sync_scroll();
+        }
+    }
+}
+
+/// Push deferred scrolls to the display in one batch: shift the shadow
+/// (and the rendered grid, which mirrors it) up by the pending count and
+/// do a single write-only push, then let the normal flush diff re-blit
+/// the newly-exposed rows and redraw the cursor.
+pub fn sync_scroll() {
+    unsafe {
+        let n = PENDING_SCROLLS;
+        if n == 0 {
+            return;
+        }
+        let rows = ROWS;
+        let cols = COLS;
+        gop::shift_up_n(n);
+        // Mirror the pushed content in RENDERED (it describes the fb).
+        let total = rows * cols;
+        rendered_cells().copy_within(n * cols..total, 0);
+        for i in (total - n * cols)..total {
+            rendered_cells()[i] = BLANK;
+        }
+        PENDING_SCROLLS = 0;
+        LAST_SYNC_TICK = crate::interrupts::TICKS.load(core::sync::atomic::Ordering::Relaxed);
+        flush();
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -473,6 +524,7 @@ pub fn clear_all() {
     unsafe {
         SGR_FG = 7;
         SGR_BG = 0;
+        sync_scroll();
         erase_all();
         CURSOR_R = 0;
         CURSOR_C = 0;
@@ -490,6 +542,15 @@ pub fn clear_all() {
 /// (scrolling) or full clears can never leave a stale inverse block that
 /// the diff can't see (blank-vs-blank).
 fn flush() {
+    unsafe {
+        // Deferred scrolls pending: the framebuffer lags the logical grid.
+        // Render nothing until `sync_scroll` pushes the batch — the diff
+        // would otherwise see every cell as changed and re-rasterise the
+        // whole screen once per line.
+        if PENDING_SCROLLS > 0 {
+            return;
+        }
+    }
     let rows = unsafe { ROWS };
     let cols = unsafe { COLS };
     let n = rows * cols;