fix: redundant compute-layout per child, framebuffer diff size test, test file cleanup

tests/framebuffer-tests.lisp

@@ -58,6 +58,29 @@
       (is (eql #\A (cell-char (aref cells 6 6))) "inside scissor draws")
       (is (eql #\space (cell-char (aref cells 1 1))) "outside scissor is clipped"))))
 
+(test flush-different-sized-fbs-handles-edge-cells
+  "flush-framebuffer handles prev and curr framebuffers of different sizes
+   without errors. Cells in the overlapping region are diffed; cells outside
+   the overlap are silently ignored (no crash on array bounds)."
+  (let* ((small-fb (make-framebuffer 5 5))
+         (large-fb (make-framebuffer 10 10))
+         (be (make-simple-backend :output-stream (make-string-output-stream))))
+    ;; Set a cell in the small one for a change in the overlapping region
+    (setf (aref small-fb 0 0) (make-cell :char #\X :fg :red))
+    ;; diff-framebuffers should use min dimensions (5,5) — no crash
+    (let ((changes (diff-framebuffers small-fb large-fb)))
+      (is (= 1 (length changes)) "one cell changed in overlap region"))
+    ;; flush-framebuffer should also handle different sizes gracefully
+    (let ((changed (flush-framebuffer small-fb large-fb be)))
+      (is (= 1 changed) "flush reports 1 changed cell"))
+    ;; Reverse: large as prev, small as curr — extra cells in prev ignored
+    (setf (aref large-fb 9 9) (make-cell :char #\Y :fg :blue))
+    (let ((changes2 (diff-framebuffers large-fb small-fb)))
+      (is (= 1 (length changes2)) "only overlapping region diffed (smaller bounds)"))
+    ;; flush should also work with shrunk framebuffer
+    (let ((changed2 (flush-framebuffer large-fb small-fb be)))
+      (is (= 1 changed2) "flush with shrunk fb reports 1 changed cell"))))
+
 (test flush-fb-copies-to-backend
   (let* ((real-be (make-simple-backend :output-stream (make-string-output-stream)))
          (fb (make-framebuffer-backend)))