fix: pass the actual previous step in the Tour show event

shepherd.js/src/tour.ts

@@ -400,10 +400,14 @@ export class Tour extends Evented {
    * @private
    */
   _showStep(step: Step) {
+    // `currentStep` is `undefined` until a step has shown, so normalize to
+    // `null` to keep `previous` consistent across every path into `show`
+    const previous = this.currentStep ?? null;
+
     this.currentStep = step;
     this.trigger('show', {
       step,
-      previous: this.currentStep
+      previous
     });
 
     step.show();

shepherd.js/test/unit/tour.spec.js

@@ -533,6 +533,51 @@ describe('Tour | Top-Level Class', function () {
         ).toBeFalsy();
       });
 
+      it('triggers show with the step that was showing before as `previous`', function () {
+        const shows = [];
+        instance.on('show', ({ step, previous }) =>
+          shows.push({ step, previous })
+        );
+
+        instance.start();
+        instance.next();
+
+        const [firstShow, secondShow] = shows;
+
+        expect(firstShow.step.id).toBe('test');
+        expect(
+          firstShow.previous,
+          'there is no previous step on the first show'
+        ).toBeNull();
+
+        expect(secondShow.step.id).toBe('test2');
+        expect(
+          secondShow.previous.id,
+          '`previous` is the step that was showing before'
+        ).toBe('test');
+        expect(
+          secondShow.previous,
+          '`previous` is not the same object as `step`'
+        ).not.toBe(secondShow.step);
+      });
+
+      it('reports `previous` as null when no step was showing before', function () {
+        // `removeStep` clears `currentStep` to `undefined` before showing the
+        // next step, so `previous` has to be normalized on that path too
+        instance.start();
+
+        let previousStep;
+        instance.on('show', ({ previous }) => (previousStep = previous));
+
+        instance.removeStep('test');
+
+        expect(instance.getCurrentStep().id).toBe('test2');
+        expect(
+          previousStep,
+          '`previous` is null rather than undefined when nothing was showing'
+        ).toBeNull();
+      });
+
       it('showOn determines which steps to skip', function () {
         instance.start();
         expect(instance.getCurrentStep().id).toBe('test');