fix(kitty-graphics): zlib-compress images forwarded to the host terminal (#5608)

* fix(kitty-graphics): zlib-compress images forwarded to the host terminal

`emit_kitty_transmit` sent every image to the host as uncompressed RGBA
base64 (`f=32,t=d`), about 20x the size of a PNG the application sent,
and re-sent it on every tab switch, pane split and resize step. Compress
the cached host payload with zlib and tag the transmit with `o=z`, which
the kitty protocol defines as part of its base feature set.

Measured with a headless client at 429x77 (8x17 px cells): a 121 KB PNG
went from 2.4 MB to 163 KB on the wire, a 943 KB PNG from 19.9 MB to
1.9 MB, with no measurable change in server CPU.

Fixes #5596

* fix: conditional zlib compression depending on outer terminal support

* rustfmt

* add pr

---------

Co-authored-by: Aram Drevekenin <[email protected]>

zellij-integration-tests/Cargo.toml

@@ -24,4 +24,5 @@ vte = { workspace = true }
 
 [dev-dependencies]
 base64 = { version = "0.22", default-features = false, features = ["std"] }
+flate2 = { version = "1.1.9", default-features = false, features = ["rust_backend"] }
 tempfile = { workspace = true }

zellij-integration-tests/src/client_screen.rs

@@ -28,6 +28,7 @@ struct ReceivedBytesWithChangeSignal {
 pub enum HostTerminal {
     Basic,
     Kitty,
+    KittyZlib,
     Manual,
 }
 
@@ -35,6 +36,8 @@ const PRIMARY_DA_QUERY: &[u8] = b"\x1b[c";
 const PRIMARY_DA_REPLY: &[u8] = b"\x1b[?62;22c";
 const KITTY_GRAPHICS_PROBE_PREFIX: &[u8] = b"\x1b_Ga=q,i=31,";
 const KITTY_GRAPHICS_PROBE_REPLY: &[u8] = b"\x1b_Gi=31;OK\x1b\\";
+const KITTY_ZLIB_PROBE_PREFIX: &[u8] = b"\x1b_Ga=q,i=32,";
+const KITTY_ZLIB_PROBE_REPLY: &[u8] = b"\x1b_Gi=32;OK\x1b\\";
 const CELL_SIZE_QUERY: &[u8] = b"\x1b[16t";
 const CELL_SIZE_REPLY: &[u8] = b"\x1b[6;21;8t";
 
@@ -53,6 +56,12 @@ impl HostResponder {
                 (KITTY_GRAPHICS_PROBE_PREFIX, KITTY_GRAPHICS_PROBE_REPLY),
                 (CELL_SIZE_QUERY, CELL_SIZE_REPLY),
             ],
+            HostTerminal::KittyZlib => &[
+                (PRIMARY_DA_QUERY, PRIMARY_DA_REPLY),
+                (KITTY_GRAPHICS_PROBE_PREFIX, KITTY_GRAPHICS_PROBE_REPLY),
+                (KITTY_ZLIB_PROBE_PREFIX, KITTY_ZLIB_PROBE_REPLY),
+                (CELL_SIZE_QUERY, CELL_SIZE_REPLY),
+            ],
             HostTerminal::Manual => &[],
         }
     }

zellij-integration-tests/tests/kitty_graphics.rs

@@ -65,6 +65,12 @@ fn start_zellij() -> TestSession {
         .start()
 }
 
+fn start_zellij_with_a_zlib_capable_host() -> TestSession {
+    TestRunner::new(TERMINAL_SIZE)
+        .with_host_terminal(HostTerminal::KittyZlib)
+        .start()
+}
+
 fn setup_kitty_host(zellij: &TestSession, terminal: &FakePtyHandle) {
     terminal.disable_echo();
     zellij.send_stdin(b"Z");
@@ -107,6 +113,41 @@ fn pane_kitty_image_reaches_client_with_transmit_and_placement() {
     zellij.quit();
 }
 
+#[test]
+fn kitty_image_is_zlib_compressed_when_the_host_confirms_zlib_support() {
+    use base64::Engine as _;
+    use std::io::Read;
+
+    let mut zellij = start_zellij_with_a_zlib_capable_host();
+    let terminal = claim_first_terminal_and_wait_for_prompt(&zellij);
+    setup_kitty_host(&zellij, &terminal);
+
+    terminal.output(RGB_2X2_A_T);
+
+    let compressed_header: &[u8] = b"\x1b_Ga=t,q=2,f=32,o=z,t=d,i=2000000000,s=2,v=2,m=0;";
+    let bytes = zellij.wait_until_raw_output(
+        "compressed kitty transmit and placement reach the client",
+        |bytes| {
+            contains_bytes(bytes, compressed_header)
+                && contains_bytes(bytes, PLACEMENT)
+                && !contains_bytes(bytes, TRANSMIT_HEADER)
+        },
+    );
+
+    let payload_start = find_bytes(&bytes, compressed_header).unwrap() + compressed_header.len();
+    let payload_len = find_bytes(&bytes[payload_start..], b"\x1b\\").unwrap();
+    let compressed = base64::engine::general_purpose::STANDARD
+        .decode(&bytes[payload_start..payload_start + payload_len])
+        .unwrap();
+    let mut inflated = Vec::new();
+    flate2::read::ZlibDecoder::new(&compressed[..])
+        .read_to_end(&mut inflated)
+        .unwrap();
+    assert_eq!(inflated, vec![0xffu8; 2 * 2 * 4]);
+
+    zellij.quit();
+}
+
 #[test]
 fn kitty_image_at_bottom_row_scrolls_and_reaches_client() {
     let mut zellij = start_zellij();