Fix annotation rectangle under a rotated transformation matrix

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Unreleased
 
 - Add a `hidden` option to form annotation methods, for a field that should start hidden (e.g. one an interactive action reveals later) instead of the usual default of visible and printable
+- Fix annotations placed under `doc.rotate()` marking the wrong area, because `_convertRect` derived each corner's y from the already transformed x and mapped only two of the four corners, so the rectangle a viewer makes interactive did not follow the rotated content. Fixes #1153
 
 ### [v0.20.2] - 2026-08-29
 

lib/mixins/annotations.js

@@ -180,19 +180,31 @@ export default {
 
   _convertRect(x1, y1, w, h) {
     // flip y1 and y2
-    let y2 = y1;
+    const y2 = y1;
     y1 += h;
 
     // make x2
-    let x2 = x1 + w;
+    const x2 = x1 + w;
 
     // apply current transformation matrix to points
     const [m0, m1, m2, m3, m4, m5] = this._ctm;
-    x1 = m0 * x1 + m2 * y1 + m4;
-    y1 = m1 * x1 + m3 * y1 + m5;
-    x2 = m0 * x2 + m2 * y2 + m4;
-    y2 = m1 * x2 + m3 * y2 + m5;
-
-    return [x1, y1, x2, y2];
+    const transform = (x, y) => [m0 * x + m2 * y + m4, m1 * x + m3 * y + m5];
+
+    // ISO 32000-1 12.5.2 defines Rect as an axis aligned rectangle in default
+    // user space, so a rotated or skewed matrix needs the bounding box of all
+    // four transformed corners. Two corners are enough only while the matrix
+    // keeps the axes aligned, and picking the extremes also keeps the result
+    // normalized ([llx lly urx ury], ISO 32000-1 7.9.5) when the matrix
+    // mirrors an axis.
+    const corners = [
+      transform(x1, y1),
+      transform(x2, y1),
+      transform(x2, y2),
+      transform(x1, y2),
+    ];
+    const xs = corners.map((corner) => corner[0]);
+    const ys = corners.map((corner) => corner[1]);
+
+    return [Math.min(...xs), Math.min(...ys), Math.max(...xs), Math.max(...ys)];
   },
 };

tests/unit/annotations.spec.js

@@ -101,6 +101,28 @@ describe('Annotations', () => {
     });
   });
 
+  describe('rectangle under a transformation matrix', () => {
+    test('covers the whole rotated area', () => {
+      const docData = logData(document);
+
+      // Turning a 100x100 box a quarter turn about its own top left corner
+      // swings it from x 100..200 across to x 0..100 and leaves it spanning
+      // y 672..772 in default user space.
+      document.rotate(90, { origin: [100, 20] });
+      document.link(100, 20, 100, 100, 'http://www.example.com');
+
+      expect(docData.join('\n')).toContain('/Rect [0 672 100 772]');
+    });
+
+    test('leaves an untransformed rectangle alone', () => {
+      const docData = logData(document);
+
+      document.link(100, 20, 100, 100, 'http://www.example.com');
+
+      expect(docData.join('\n')).toContain('/Rect [100 672 200 772]');
+    });
+  });
+
   describe('undefined option values', () => {
     // `doc.annotate()` passes arbitrary dictionary keys straight through by
     // design, so unlike the acroform options there is no call site at which an