diff --git a/tests/test_splitter_drag.mjs b/tests/test_splitter_drag.mjs
new file mode 100644
index 0000000..a46b4eb
--- /dev/null
+++ b/tests/test_splitter_drag.mjs
@@ -0,0 +1,136 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { startSplitterDrag } from '../web/resolver/utils/splitter_drag.js';
+
+function installDragEnvironment() {
+  const previous = {
+    document: globalThis.document,
+    requestAnimationFrame: globalThis.requestAnimationFrame,
+    cancelAnimationFrame: globalThis.cancelAnimationFrame,
+  };
+  const listeners = new Map();
+  const frames = [];
+  globalThis.document = {
+    addEventListener(type, handler) {
+      listeners.set(type, handler);
+    },
+    removeEventListener(type, handler) {
+      if (listeners.get(type) === handler) listeners.delete(type);
+    },
+  };
+  globalThis.requestAnimationFrame = callback => {
+    frames.push(callback);
+    return frames.length;
+  };
+  globalThis.cancelAnimationFrame = () => {};
+
+  return {
+    listeners,
+    frames,
+    restore() {
+      globalThis.document = previous.document;
+      globalThis.requestAnimationFrame = previous.requestAnimationFrame;
+      globalThis.cancelAnimationFrame = previous.cancelAnimationFrame;
+    },
+  };
+}
+
+function pointerEvent(type, clientX, button = undefined) {
+  return {
+    type,
+    clientX,
+    ...(button === undefined ? {} : { button }),
+    preventDefault() {},
+    stopPropagation() {},
+  };
+}
+
+test('splitter drag rejects non-primary buttons and completes a mouse drag with bounds', () => {
+  const env = installDragEnvironment();
+  try {
+    assert.equal(startSplitterDrag(pointerEvent('mousedown', 100, 2), { startWidth: 300 }), null);
+
+    const layouts = [];
+    const ended = [];
+    startSplitterDrag(pointerEvent('mousedown', 100, 0), {
+      anchor: 'right',
+      startWidth: 300,
+      bounds: { min: 200, max: 500 },
+      onDrag: width => layouts.push(width),
+      onEnd: (width, state) => ended.push({ width, ...state }),
+    });
+    env.listeners.get('mousemove')(pointerEvent('mousemove', -500));
+    env.frames.shift()();
+    env.listeners.get('mouseup')(pointerEvent('mouseup', -500));
+
+    assert.deepEqual(layouts, [500]);
+    assert.equal(ended[0].width, 500);
+    assert.equal(ended[0].didDrag, true);
+  } finally {
+    env.restore();
+  }
+});
+
+test('splitter drag applies custom width normalization and preserves threshold clicks', () => {
+  const env = installDragEnvironment();
+  try {
+    const layouts = [];
+    const ended = [];
+    startSplitterDrag(pointerEvent('mousedown', 100, 0), {
+      anchor: 'left',
+      startWidth: 300,
+      dragThreshold: 5,
+      layoutFrameStride: 1,
+      onBeforeDrag: width => width + 0.4,
+      onDrag: width => layouts.push(width),
+      onEnd: (width, state) => ended.push({ width, ...state }),
+    });
+    env.listeners.get('mousemove')(pointerEvent('mousemove', 103));
+    assert.equal(env.frames.length, 0);
+    env.listeners.get('mousemove')(pointerEvent('mousemove', 110));
+    env.frames.shift()();
+    env.listeners.get('mouseup')(pointerEvent('mouseup', 110));
+
+    assert.deepEqual(layouts, [310]);
+    assert.equal(ended[0].didDrag, true);
+
+    const noDragEnd = [];
+    startSplitterDrag(pointerEvent('mousedown', 50, 0), {
+      startWidth: 250,
+      onEnd: (width, state) => noDragEnd.push({ width, ...state }),
+    });
+    env.listeners.get('mouseup')(pointerEvent('mouseup', 50));
+    assert.deepEqual(noDragEnd, [{ width: 250, didDrag: false, appliedWidth: 250 }]);
+  } finally {
+    env.restore();
+  }
+});
+
+test('splitter drag cancel removes listeners and reports the applied preview width', () => {
+  const env = installDragEnvironment();
+  try {
+    const previews = [];
+    const drag = startSplitterDrag(pointerEvent('pointerdown', 100, 0), {
+      anchor: 'left',
+      startWidth: 300,
+      onPreview: (pending, applied, state) => previews.push({ pending, applied, ...state }),
+    });
+    env.listeners.get('pointermove')(pointerEvent('pointermove', 130));
+    const scheduledFrame = env.frames.shift();
+    scheduledFrame();
+    drag.cancel();
+
+    assert.equal(env.listeners.has('pointermove'), false);
+    assert.equal(env.listeners.has('pointerup'), false);
+    assert.equal(env.listeners.has('pointercancel'), false);
+    assert.deepEqual(previews.at(-1), {
+      pending: 330,
+      applied: 330,
+      cancelled: true,
+      final: true,
+    });
+  } finally {
+    env.restore();
+  }
+});