diff --git a/src/viewport-three/viewport-host.ts b/src/viewport-three/viewport-host.ts
index cd146b6c..6fe5734e 100644
--- a/src/viewport-three/viewport-host.ts
+++ b/src/viewport-three/viewport-host.ts
@@ -5106,6 +5106,10 @@ export class ViewportHost {
     );
 
     if (transformPointerIntent.commitActiveTransform) {
+      if (this.currentTransformSession.kind !== "active") {
+        throw new Error("Active transform intent resolved without an active session.");
+      }
+
       event.preventDefault();
       this.transformCommitHandler?.(this.currentTransformSession);
       return;
diff --git a/tests/unit/transform-pointer-intent.test.ts b/tests/unit/transform-pointer-intent.test.ts
new file mode 100644
index 00000000..1f2fa284
--- /dev/null
+++ b/tests/unit/transform-pointer-intent.test.ts
@@ -0,0 +1,78 @@
+import { describe, expect, it } from "vitest";
+
+import { createBoxBrush } from "../../src/document/brushes";
+import { resolveTransformPointerDownIntent } from "../../src/viewport-three/transform-pointer-intent";
+import type { TransformSessionState } from "../../src/core/transform-session";
+
+function createActiveBrushTransformSession(): TransformSessionState {
+  const brush = createBoxBrush({
+    id: "brush-transform-pointer-intent",
+    center: {
+      x: 0,
+      y: 1,
+      z: 0
+    }
+  });
+
+  return {
+    kind: "active",
+    id: "transform-session-pointer-intent",
+    source: "keyboard",
+    sourcePanelId: "topLeft",
+    operation: "scale",
+    axisConstraint: null,
+    axisConstraintSpace: "world",
+    target: {
+      kind: "brush",
+      brushId: brush.id,
+      initialCenter: brush.center,
+      initialRotationDegrees: brush.rotationDegrees,
+      initialSize: brush.size,
+      initialGeometry: brush.geometry
+    },
+    preview: {
+      kind: "brush",
+      center: brush.center,
+      rotationDegrees: brush.rotationDegrees,
+      size: {
+        x: brush.size.x * 1.5,
+        y: brush.size.y,
+        z: brush.size.z
+      },
+      geometry: brush.geometry
+    }
+  };
+}
+
+describe("resolveTransformPointerDownIntent", () => {
+  it("commits an active transform before allowing gizmo interaction in the same panel", () => {
+    expect(
+      resolveTransformPointerDownIntent(
+        createActiveBrushTransformSession(),
+        "topLeft"
+      )
+    ).toEqual({
+      commitActiveTransform: true,
+      allowGizmoInteraction: false
+    });
+  });
+
+  it("blocks pointer handling in other panels while a different panel owns the active transform", () => {
+    expect(
+      resolveTransformPointerDownIntent(
+        createActiveBrushTransformSession(),
+        "topRight"
+      )
+    ).toEqual({
+      commitActiveTransform: false,
+      allowGizmoInteraction: false
+    });
+  });
+
+  it("allows gizmo interaction when no transform session is active", () => {
+    expect(resolveTransformPointerDownIntent({ kind: "none" }, "topLeft")).toEqual({
+      commitActiveTransform: false,
+      allowGizmoInteraction: true
+    });
+  });
+});