layout/layout.lisp

@@ -76,7 +76,8 @@
   "Compute child sizes given available space and gap.
 HORIZONTAL is non-nil when distributing width (row layout).
 Each child starts from its fixed size (if any). Remaining space
-is distributed by grow ratio; overflow is reduced by shrink ratio."
+is distributed by grow ratio; overflow is reduced by shrink ratio.
+Rounding errors are amortized across the first N children."
   (let* ((n (length children))
          (gap-total (* gap (max 0 (1- n))))
          (base (mapcar (lambda (c)
@@ -89,14 +90,25 @@ is distributed by grow ratio; overflow is reduced by shrink ratio."
          (remaining (- avail base-total gap-total))
          (grow-total (reduce #'+ (mapcar #'layout-node-grow children)))
          (shrink-total (reduce #'+ (mapcar #'layout-node-shrink children))))
-    (mapcar (lambda (c b)
-              (let ((sz b))
-                (when (and (plusp remaining) (plusp grow-total))
-                  (incf sz (round (* remaining (/ (layout-node-grow c) grow-total)))))
-                (when (and (minusp remaining) (plusp shrink-total))
-                  (decf sz (round (* (abs remaining) (/ (layout-node-shrink c) shrink-total)))))
-                (max 1 sz)))
-            children base)))
+    (let ((sizes (mapcar (lambda (c b)
+                           (let ((sz b))
+                             (when (and (plusp remaining) (plusp grow-total))
+                               (incf sz (round (* remaining (/ (layout-node-grow c) grow-total)))))
+                             (when (and (minusp remaining) (plusp shrink-total))
+                               (decf sz (round (* (abs remaining) (/ (layout-node-shrink c) shrink-total)))))
+                             (max 1 sz)))
+                         children base)))
+      ;; Distribute rounding remainder to first N children so that
+      ;; the total of sizes exactly fills avail minus gap-total.
+      ;; Only correct when grow or shrink was actually applied —
+      ;; otherwise children keep their fixed sizes and may not fill space.
+      (when (or (and (plusp remaining) (plusp grow-total))
+                (and (minusp remaining) (plusp shrink-total)))
+        (let ((delta (- avail gap-total (reduce #'+ sizes))))
+          (when (/= delta 0)
+            (loop :for i :from 0 :below (min (abs delta) n)
+                  :do (incf (nth i sizes) (signum delta))))))
+      sizes)))
 
 (defun compute-layout (root available-width available-height)
   "Layout all children of ROOT within the given dimensions.

src/components/render.lisp

@@ -32,9 +32,9 @@
 (defun render-screen (root backend)
   "Render the component tree ROOT using BACKEND.
   Computes layout for dirty branches, calls render on each component,
-  and wraps output in synchronized updates."
-  (let ((w (available-width root))
-        (h (available-height root)))
+  and wraps output in synchronized updates. Uses the actual terminal
+  dimensions from BACKEND rather than hardcoded defaults."
+  (multiple-value-bind (w h) (backend-size backend)
     (begin-sync backend)
     (render-node root backend w h)
     (end-sync backend)))