Fix pager rendering slowness.

src/Session.zig

@@ -38,6 +38,11 @@ watch: c_uint = 0,
 /// True once the child process has exited and the PTY hung up.
 exited: bool = false,
 
+/// Cell size in pixels, as last set by a resize. Kept because the XTWINOPS
+/// size queries report it and the answer has to come from somewhere; the
+/// widget owns the real geometry and hands it down here.
+cell_px: struct { w: u32 = 8, h: u32 = 16 } = .{},
+
 /// Latest state reported by the child. Kept so a repeated report of the
 /// state we are already in doesn't wake the UI for nothing.
 status: Status = .idle,
@@ -139,13 +144,23 @@ pub fn create(
     errdefer self.stream.deinit();
 
     // Wire up the side effects we care about. `readonly` handles terminal
-    // state but silently drops anything that needs to talk back to the
-    // child; we need the writes so that queries (cursor position, device
-    // attributes, in-band resize) get answers.
+    // state but silently drops anything that needs to talk back to the child.
+    //
+    // Every query below has to be answered, not merely permitted to write.
+    // `write_pty` is only the pipe; a query whose own effect is null is
+    // dropped before it ever reaches that pipe. And an unanswered query is
+    // not a missing feature, it is a stall: the standard way to test for a
+    // capability is to send the query followed by a device attributes
+    // request, and take the DA1 reply arriving alone as "unsupported".
+    // A terminal that answers neither leaves the program waiting out its
+    // own timeout, once per run, for every capability it probes.
     var effects = vt.TerminalStream.Handler.Effects.readonly;
     effects.write_pty = &effectWritePty;
     effects.title_changed = &effectTitleChanged;
     effects.progress_report = &effectProgressReport;
+    effects.device_attributes = &effectDeviceAttributes;
+    effects.color_scheme = &effectColorScheme;
+    effects.size = &effectSize;
     self.stream.handler.effects = effects;
 
     const shell = try defaultShell(alloc);
@@ -268,6 +283,48 @@ fn effectWritePty(handler: *vt.TerminalStream.Handler, data: [:0]const u8) void
 }
 
 /// Effect callback: OSC 0/2 changed the window title.
+/// The reply type an effect has to return, recovered from the effect's own
+/// signature. libghostty-vt exports the device attributes *request* enum by
+/// name but not the reply struct, and naming it this way can't drift from
+/// what the callback is actually required to return.
+fn EffectReply(comptime name: []const u8) type {
+    const Opt = @FieldType(vt.TerminalStream.Handler.Effects, name);
+    const Fn = @typeInfo(@typeInfo(Opt).optional.child).pointer.child;
+    return @typeInfo(Fn).@"fn".return_type.?;
+}
+
+/// Effect callback: the child asked what kind of terminal this is (CSI c and
+/// friends). The defaults describe a VT220 that does ANSI colour, which is
+/// what this actually is — the answer matters less than there being one.
+fn effectDeviceAttributes(
+    _: *vt.TerminalStream.Handler,
+) EffectReply("device_attributes") {
+    return .{};
+}
+
+/// Effect callback: the child asked whether it is drawing on a light or dark
+/// background (CSI ? 996 n), so it can pick readable colours. Answered from
+/// the scheme the grid is actually painted in.
+fn effectColorScheme(
+    _: *vt.TerminalStream.Handler,
+) ?vt.device_status.ColorScheme {
+    return switch (theme.currentScheme()) {
+        .light => .light,
+        .dark => .dark,
+    };
+}
+
+/// Effect callback: the child asked for the terminal geometry (CSI 14/16/18 t).
+fn effectSize(handler: *vt.TerminalStream.Handler) ?vt.size_report.Size {
+    const self = fromHandler(handler);
+    return .{
+        .rows = self.term.rows,
+        .columns = self.term.cols,
+        .cell_width = self.cell_px.w,
+        .cell_height = self.cell_px.h,
+    };
+}
+
 fn effectTitleChanged(handler: *vt.TerminalStream.Handler) void {
     const self = fromHandler(handler);
     self.on_title(self.ctx, self.term.title.items);
@@ -320,6 +377,10 @@ pub fn write(self: *Session, bytes: []const u8) void {
 
 /// Resize the terminal grid and tell the child about it.
 pub fn resize(self: *Session, cols: u16, rows: u16, cell_w: u32, cell_h: u32) !void {
+    // Recorded before the early return: the grid can keep its dimensions
+    // across a font change that moves every pixel measurement.
+    self.cell_px = .{ .w = cell_w, .h = cell_h };
+
     if (cols == self.term.cols and rows == self.term.rows) return;
 
     try self.stream.handler.resize(.{