A detached session no client watches draws no frames: yes | head -c 50M through pty/run goes from over 30 s to 11 s

Profiling the 50 MB run showed 70% of its time in draw.render,
recolorAnsi alone 43%, walking cells through PageList.pin. The detached
session drew a frame after every pty read, with no client attached to
see it. A host now says whether anyone watches (Host.VTable.watched). The
detached server answers whether a client is attached, and while none is
the core keeps the frame owed instead of drawing it. A /screen read draws
its own, and a client attaching gets one (hello sets needs_frame).

Debug, 80x24: 10 MB went from 7.36 s to 2.10 s, and 50 MB from over the
30 s client timeout (about 37 s by rate) to 11.08 s. What is left is the
terminal's own parsing and scrollback growth. A session a frontend shows
still draws after every read. Pacing those frames is the next step if
that case matters.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/detached/server.zig b/src/detached/server.zig
index d8379793..001acc47 100644
--- a/src/detached/server.zig
+++ b/src/detached/server.zig
@@ -376,6 +376,7 @@ pub const Session = struct {
         .now = clockNow,
         .present = present,
         .poll_frame = pollFrame,
+        .watched = watched,
         .spawn = spawn,
         .pty_write = ptyWrite,
         .pty_resize = ptyResize,
@@ -641,6 +642,12 @@ pub const Session = struct {
         s.mailbox.signal();
     }
 
+    fn watched(ctx: ?*anyopaque) bool {
+        const s = of(ctx);
+        for (&s.clients) |*c| if (c.attached) return true;
+        return false;
+    }
+
     fn pollFrame(ctx: ?*anyopaque) void {
         const s = of(ctx);
         if (s.ninep) |l| s.ninep_pending = l.tick().pending;
@@ -1052,6 +1059,9 @@ pub const Session = struct {
                 c.attached = true;
                 c.need_full = true;
                 c.greet = true;
+                // Unwatched until now, it may have drawn nothing: a frame
+                // for the one attaching (watched).
+                s.core.needs_frame = true;
             },
             .bye => s.close(c, .bye),
             .event => |ev| {