From 8f700e7f66f8ab31ffc889f3779b044dba23f4dc Mon Sep 17 00:00:00 2001
From: Greg Pomerantz <gmp@wow.st>
Date: Sun, 1 Nov 2026 17:44:14 -0500
Subject: [PATCH] gtauri: clamp viewer pan/zoom to image edges

Pan is now limited to the image bounds (|offset| <= (rendered -
viewport)/2 per axis), so the image can never be dragged off-screen
leaving blank space around it; on a letterboxed axis panning is
locked to centered. Pinch can no longer zoom out past fit. Clamping
lives in vApply() so pan, pinch and reset all share it; still
compositor-only (transform only).
---
 harnesses/gtauri/frontend/dist/index.html | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/harnesses/gtauri/frontend/dist/index.html b/harnesses/gtauri/frontend/dist/index.html
index a82a890..1750e0b 100644
--- a/harnesses/gtauri/frontend/dist/index.html
+++ b/harnesses/gtauri/frontend/dist/index.html
@@ -269,7 +269,21 @@ requestAnimationFrame(frTick);
 let vScale = 1, vTx = 0, vTy = 0;
 const base = { w: 0, h: 0 };
 
+function fitScale() {
+  return Math.min(innerWidth / base.w, innerHeight / base.h);
+}
+
 function vApply() {
+  // Clamp: never zoom out past fit, and pan is limited to the image
+  // edges — the image can never be dragged off-screen leaving blank
+  // space around it. On a letterboxed axis (image smaller than the
+  // viewport in that dimension) panning is locked to centered.
+  const s = fitScale();
+  if (vScale < s) vScale = s;
+  const mx = Math.max(0, (base.w * vScale - innerWidth) / 2);
+  const my = Math.max(0, (base.h * vScale - innerHeight) / 2);
+  if (vTx > mx) vTx = mx; else if (vTx < -mx) vTx = -mx;
+  if (vTy > my) vTy = my; else if (vTy < -my) vTy = -my;
   vimg.style.transform =
     "translate(" + (vTx - base.w * vScale / 2) + "px," + (vTy - base.h * vScale / 2) + "px) scale(" + vScale + ")";
 }
@@ -370,7 +384,7 @@ viewer.addEventListener("pointermove", (e) => {
     p.x = e.clientX; p.y = e.clientY;
     const [a, b] = [...pts.values()];
     const d = Math.hypot(a.x - b.x, a.y - b.y);
-    if (pinchD0 > 0) vScale = Math.max(0.05, pinchS0 * d / pinchD0);
+    if (pinchD0 > 0) vScale = Math.max(fitScale(), pinchS0 * d / pinchD0);
     vApply();
   }
 });
@@ -378,7 +392,7 @@ const endPt = (e) => { pts.delete(e.pointerId); };
 viewer.addEventListener("pointerup", endPt);
 viewer.addEventListener("pointercancel", endPt);
 viewer.addEventListener("dblclick", () => {
-  vScale = Math.min(innerWidth / base.w, innerHeight / base.h);
+  vScale = fitScale();
   vTx = 0; vTy = 0; vApply();
 });
 </script>