From 5a3b882f93074614f2f12ea06c1518839684dab4 Mon Sep 17 00:00:00 2001
From: Amr Gharbeia <amr@gharbeia.net>
Date: Thu, 14 May 2026 10:14:40 -0400
Subject: [PATCH] fix: add blocking-read-based CSI 18t terminal size query
 fallback

%query-terminal-size uses blocking read-char on an fd 0 stream
to read the terminal's response to \033[18t. This works even when
unix-simple-poll on fd 0 returns NIL (unlike read-char-no-hang).
Added as fallback in both modern and simple backends.
---
 src/backend/classes.lisp | 32 ++++++++++++++++++++++++++++++++
 src/backend/modern.lisp  |  1 +
 src/backend/simple.lisp  |  1 +
 3 files changed, 34 insertions(+)

diff --git a/src/backend/classes.lisp b/src/backend/classes.lisp
index 677a565..1e4af64 100644
--- a/src/backend/classes.lisp
+++ b/src/backend/classes.lisp
@@ -1,5 +1,37 @@
 (in-package :cl-tty.backend)
 
+(defun %query-terminal-size ()
+  "Query terminal size via ANSI CSI 18 t using blocking read.
+Returns (values cols rows) or nil."
+  (ignore-errors
+    (let* ((saved (sb-sys:make-fd-stream 0 :input t :buffering :none))
+           (response (make-array 0 :element-type 'character
+                                   :fill-pointer 0 :adjustable t)))
+      (format t "~C[18t" #\Esc)
+      (force-output)
+      ;; Blocking read-char loop — the response arrives immediately
+      (loop with deadline = (+ (get-internal-real-time)
+                               (* internal-time-units-per-second 0.5))
+            while (< (get-internal-real-time) deadline)
+            do (let ((ch (read-char saved nil nil)))
+                 (when ch
+                   (vector-push-extend ch response)
+                   (when (char= ch #\t) (return)))))
+      (when (>= (length response) 8)
+        (let* ((str (subseq response 1))
+               (start (or (position #\[ str) 0))
+               (after (subseq str (1+ start)))
+               (semi (position #\; after)))
+          (when semi
+            (let* ((cols-start (1+ semi))
+                   (cols-end (position #\t after :start cols-start))
+                   (rows (parse-integer (subseq after 0 semi) :junk-allowed t))
+                   (cols (when cols-end
+                           (parse-integer (subseq after cols-start cols-end)
+                                          :junk-allowed t))))
+              (when (and rows cols (> rows 0) (> cols 0))
+                (values cols rows)))))))))
+
 (defclass backend () ())
 
 (defgeneric initialize-backend (backend)
diff --git a/src/backend/modern.lisp b/src/backend/modern.lisp
index 4f975a8..792834b 100644
--- a/src/backend/modern.lisp
+++ b/src/backend/modern.lisp
@@ -175,6 +175,7 @@ as a fallback when a keyword is not in *named-colors*.")
                  (values (sb-alien:deref winsize 1)  ;; cols
                          (sb-alien:deref winsize 0))) ;; rows
             (sb-alien:free-alien winsize))))
+      (%query-terminal-size)
       (values 80 24)))
 
 (defmethod backend-write ((b modern-backend) string)
diff --git a/src/backend/simple.lisp b/src/backend/simple.lisp
index 0ca931f..e00c091 100644
--- a/src/backend/simple.lisp
+++ b/src/backend/simple.lisp
@@ -39,6 +39,7 @@
                  (values (sb-alien:deref winsize 1)
                          (sb-alien:deref winsize 0)))
              (sb-alien:free-alien winsize))))
+      (%query-terminal-size)
       (values 80 24)))
 
 (defmethod backend-write ((b simple-backend) string)