From d1e93d4cdc2e2625d5a1724a7167a5c3e64fe6a2 Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Tue, 29 Sep 2026 19:49:19 -0300
Subject: Over a slow link the tty keeps one frame in flight: it asks the
 terminal to answer after each, and draws the latest state when it does

---
 test/e2e_harness.zig | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

(limited to 'test/e2e_harness.zig')

diff --git a/test/e2e_harness.zig b/test/e2e_harness.zig
index eaa19b43..9e67c49f 100644
--- a/test/e2e_harness.zig
+++ b/test/e2e_harness.zig
@@ -64,6 +64,8 @@ pub const Harness = struct {
     /// when true, print the captured screen state after each pump/waitFor and on
     /// every assertion, so live test runs can be inspected (zig build test -Dtrace).
     trace: bool = false,
+    /// How much of a `CSI 5 n` the output has ended on (`feed`).
+    status_request: u8 = 0,
     /// every raw byte the app has emitted (accumulated in pump). Lets tests assert
     /// on control sequences the emulator consumes and never renders (e.g. OSC 52
     /// clipboard writes). Capture is bounded explicitly so a runaway child cannot
@@ -147,6 +149,24 @@ pub const Harness = struct {
 
     /// Read pty output and feed it to our ghostty terminal for `ms` ms. After
     /// this, the grid reflects everything the app rendered so far.
+    /// The app's output into the emulator. The one query answered is the
+    /// device status request (`CSI 5 n` → `CSI 0 n`), as every terminal
+    /// does: the tty asks it after each frame and draws no other until the
+    /// answer (FrameAck in src/tty/tty.zig).
+    fn feed(self: *Harness, bytes: []const u8) void {
+        self.stream.nextSlice(bytes);
+        const request = "\x1b[5n";
+        for (bytes) |b| {
+            if (b == request[self.status_request]) {
+                self.status_request += 1;
+            } else self.status_request = if (b == 0x1b) 1 else 0;
+            if (self.status_request == request.len) {
+                self.status_request = 0;
+                _ = libc.write(self.master, "\x1b[0n", 4);
+            }
+        }
+    }
+
     pub fn pump(self: *Harness, ms: i64) !void {
         const deadline = nowMs() + ms;
         var buf: [4096]u8 = undefined;
@@ -157,7 +177,7 @@ pub const Harness = struct {
                 const n = posix.read(self.master, &buf) catch break;
                 if (n == 0) break;
                 self.recordRaw(buf[0..n]);
-                self.stream.nextSlice(buf[0..n]);
+                self.feed(buf[0..n]);
             }
         }
         if (self.trace) self.traceScreen("pump");
@@ -176,7 +196,7 @@ pub const Harness = struct {
         const n = posix.read(self.master, &buf) catch return false;
         if (n == 0) return false;
         self.recordRaw(buf[0..n]);
-        self.stream.nextSlice(buf[0..n]);
+        self.feed(buf[0..n]);
         return true;
     }
 
@@ -243,7 +263,7 @@ pub const Harness = struct {
                 const n = posix.read(self.master, &buf) catch break;
                 if (n == 0) break;
                 self.recordRaw(buf[0..n]);
-                self.stream.nextSlice(buf[0..n]);
+                self.feed(buf[0..n]);
             }
             const text = try self.screenText();
             defer self.gpa.free(text);
@@ -270,7 +290,7 @@ pub const Harness = struct {
             const n = posix.read(self.master, &buf) catch break;
             if (n == 0) break;
             self.recordRaw(buf[0..n]);
-            self.stream.nextSlice(buf[0..n]);
+            self.feed(buf[0..n]);
             if (std.mem.indexOf(u8, self.raw.items, needle) != null) return true;
         }
         return false;
-- 
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