diff --git a/scripts/format-scope.json b/scripts/format-scope.json
index caffd41d2..cc248f38d 100644
--- a/scripts/format-scope.json
+++ b/scripts/format-scope.json
@@ -147,6 +147,7 @@
   "src/ui/panelRails.js",
   "src/ui/leftPanelRail.js",
   "src/ui/rightPanelRail.js",
+  "src/ui/panelResize.test.mjs",
   "src/ui/panelMeasurement.js",
   "src/ui/panelRailGeometry.js",
   "src/ui/panelRails.test.mjs",
diff --git a/scripts/package-boundaries.json b/scripts/package-boundaries.json
index 433f5f36a..8e6cda2cd 100644
--- a/scripts/package-boundaries.json
+++ b/scripts/package-boundaries.json
@@ -553,6 +553,7 @@
       "src/ui/panelLayoutController.js",
       "src/ui/panelMeasurement.js",
       "src/ui/panelPositionControls.js",
+      "src/ui/panelResize.js",
       "src/ui/panelRailGeometry.js",
       "src/ui/panelRails.js",
       "src/ui/radio.js",
@@ -1670,6 +1671,7 @@
       "src/ui/panelLayoutController.js",
       "src/ui/panelMeasurement.js",
       "src/ui/panelPositionControls.js",
+      "src/ui/panelResize.js",
       "src/ui/panelRailGeometry.js",
       "src/ui/panelRails.js",
       "src/ui/radio.js",
@@ -2327,6 +2329,7 @@
       "src/ui/panelLayoutController.js",
       "src/ui/panelMeasurement.js",
       "src/ui/panelPositionControls.js",
+      "src/ui/panelResize.js",
       "src/ui/panelRailGeometry.js",
       "src/ui/panelRails.js",
       "src/ui/rightPanelRail.js",
diff --git a/src/ui/panelResize.js b/src/ui/panelResize.js
new file mode 100644
index 000000000..4472e7a01
--- /dev/null
+++ b/src/ui/panelResize.js
@@ -0,0 +1,65 @@
+/**
+ * Pure geometry for resizing a floating panel from any edge or corner. The
+ * DOM wiring lives in PanelPositionControls; this module only decides boxes.
+ */
+
+/** Viewport margin a resized panel keeps clear on every side. */
+const EDGE_MARGIN_PX = 6;
+/** Resize directions: compass edges first, then corners. */
+export const RESIZE_DIRECTIONS = ['n', 's', 'e', 'w', 'ne', 'nw', 'sw', 'se'];
+
+function clamp(value, min, max) {
+  return Math.max(min, Math.min(max, value));
+}
+
+/**
+ * Work out the box a resize gesture produces. Edges named in `dir` move with
+ * the pointer; the opposite edge stays put, so growing from the left keeps
+ * the right side pinned. Minimum size and the viewport margin win over the
+ * pointer.
+ * @param {{left: number, top: number, width: number, height: number}} box Box at gesture start.
+ * @param {string} dir One of RESIZE_DIRECTIONS.
+ * @param {number} dx Pointer travel since gesture start.
+ * @param {number} dy Pointer travel since gesture start.
+ * @param {object} [limits]
+ * @param {number} limits.minWidth
+ * @param {number} limits.minHeight
+ * @param {number} [limits.viewportWidth=Infinity]
+ * @param {number} [limits.viewportHeight=Infinity]
+ * @param {number} [limits.margin=6]
+ * @returns {{left: number, top: number, width: number, height: number}}
+ */
+export function resizeBox(
+  box,
+  dir,
+  dx,
+  dy,
+  {
+    minWidth,
+    minHeight,
+    viewportWidth = Infinity,
+    viewportHeight = Infinity,
+    margin = EDGE_MARGIN_PX,
+  } = {},
+) {
+  let { left, top, width, height } = box;
+  const right = box.left + box.width;
+  const bottom = box.top + box.height;
+  if (dir.includes('e'))
+    width = clamp(box.width + dx, minWidth, viewportWidth - margin - box.left);
+  if (dir.includes('s'))
+    height = clamp(
+      box.height + dy,
+      minHeight,
+      viewportHeight - margin - box.top,
+    );
+  if (dir.includes('w')) {
+    width = clamp(box.width - dx, minWidth, right - margin);
+    left = right - width;
+  }
+  if (dir.includes('n')) {
+    height = clamp(box.height - dy, minHeight, bottom - margin);
+    top = bottom - height;
+  }
+  return { left, top, width, height };
+}
diff --git a/src/ui/panelResize.test.mjs b/src/ui/panelResize.test.mjs
new file mode 100644
index 000000000..3e1577665
--- /dev/null
+++ b/src/ui/panelResize.test.mjs
@@ -0,0 +1,64 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { resizeBox } from './panelResize.js';
+
+const box = { left: 100, top: 100, width: 400, height: 300 };
+const limits = {
+  minWidth: 200,
+  minHeight: 150,
+  viewportWidth: 1000,
+  viewportHeight: 800,
+};
+
+test('east and south edges grow away from the anchored corner', () => {
+  assert.deepEqual(resizeBox(box, 'se', 50, 20, limits), {
+    left: 100,
+    top: 100,
+    width: 450,
+    height: 320,
+  });
+  assert.deepEqual(resizeBox(box, 'e', -30, 999, limits), {
+    left: 100,
+    top: 100,
+    width: 370,
+    height: 300,
+  });
+});
+
+test('west and north edges keep the opposite edge pinned', () => {
+  assert.deepEqual(resizeBox(box, 'w', -40, 0, limits), {
+    left: 60,
+    top: 100,
+    width: 440,
+    height: 300,
+  });
+  assert.deepEqual(resizeBox(box, 'nw', 40, 30, limits), {
+    left: 140,
+    top: 130,
+    width: 360,
+    height: 270,
+  });
+  assert.deepEqual(
+    resizeBox(box, 'n', -50, 0, limits).top,
+    100,
+    'horizontal motion does not move a north edge',
+  );
+});
+
+test('minimum size stops the moving edge, not the anchored one', () => {
+  const small = resizeBox(box, 'w', 350, 0, limits);
+  assert.deepEqual(small, { left: 300, top: 100, width: 200, height: 300 });
+  assert.equal(resizeBox(box, 'ne', 0, 400, limits).height, 150);
+  assert.equal(resizeBox(box, 'ne', 0, 400, limits).top, 250);
+});
+
+test('the viewport margin caps growth in every direction', () => {
+  assert.equal(resizeBox(box, 'e', 2000, 0, limits).width, 1000 - 6 - 100);
+  assert.equal(resizeBox(box, 's', 0, 2000, limits).height, 800 - 6 - 100);
+  const west = resizeBox(box, 'w', -2000, 0, limits);
+  assert.equal(west.left, 6);
+  assert.equal(west.left + west.width, 500);
+  const north = resizeBox(box, 'n', 0, -2000, limits);
+  assert.equal(north.top, 6);
+  assert.equal(north.top + north.height, 400);
+});