fix attributes and properties turning to null from 'morphing'

docs/pages/resources/changelog.md

@@ -17,6 +17,7 @@ New versions of Shoelace are released as-needed and generally occur when a criti
 - Scroll buttons for `<sl-tab-group>` auto hide when they are not clickable. The `fixed-scroll-controls` attribute can be included to prevent this behavior. [#2128]
 - Added support for using `<sl-dropdown>` in `<sl-breadcrumb-item>` default slot [#2015]
 - Added the `countdown` attribute to `<sl-alert>` to show a visual indicator before the toast disappears [#1899]
+- Fixed a bug with morphing and DOM diffing that would cause elements with reflected initial attributes to not reset. [#2177]
 - Fixed a bug that caused errors to show in the console when components disconnect before before `firstUpdated()` executes [#2127]
 - Fixed a bug that made pagination work incorrectly in `<sl-carousel>` [#2155]
 - Fixed a bug in `<sl-tab-group>` that caused the active tab indicator to be the wrong size when the tab's content changes [#2164]

src/components/button-group/button-group.test.ts

@@ -89,7 +89,6 @@ describe('<sl-button-group>', () => {
 
       allButtons[0].dispatchEvent(new MouseEvent('mouseout', { bubbles: true }));
       await elementUpdated(allButtons[0]);
-      console.log(allButtons[0]);
       expect(allButtons[0]).to.not.have.attribute('data-sl-button-group__button--hover');
     });
   });

src/components/button/button.test.ts

@@ -15,6 +15,63 @@ describe('<sl-button>', () => {
       });
     });
   });
+  describe('when an attribute is removed', () => {
+    it("should return to 'default' when attribute removed with no initial attribute", async () => {
+      const el = await fixture<SlButton>(html`<sl-button>Button label</sl-button>`);
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+
+      el.removeAttribute('variant');
+      await el.updateComplete;
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+    });
+
+    it("should return to 'default' when attribute removed with an initial attribute", async () => {
+      const el = await fixture<SlButton>(html`<sl-button variant="primary">Button label</sl-button>`);
+
+      expect(el.variant).to.equal('primary');
+      expect(el.getAttribute('variant')).to.equal('primary');
+
+      el.removeAttribute('variant');
+      await el.updateComplete;
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+    });
+  });
+
+  describe('when a property is set to null', () => {
+    it("should return to 'default' when property set to null with no initial attribute", async () => {
+      const el = await fixture<SlButton>(html`<sl-button>Button label</sl-button>`);
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+
+      // @ts-expect-error Its a test. Stop.
+      el.variant = null;
+      await el.updateComplete;
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+    });
+
+    it("should return to 'default' when property set to null with an initial attribute", async () => {
+      const el = await fixture<SlButton>(html`<sl-button variant="primary">Button label</sl-button>`);
+
+      expect(el.variant).to.equal('primary');
+      expect(el.getAttribute('variant')).to.equal('primary');
+
+      // @ts-expect-error Its a test. Stop.
+      el.variant = null;
+      await el.updateComplete;
+
+      expect(el.variant).to.equal('default');
+      expect(el.getAttribute('variant')).to.equal('default');
+    });
+  });
 
   describe('when provided no parameters', () => {
     it('passes accessibility test', async () => {

src/internal/shoelace-element.ts

@@ -146,6 +146,42 @@ export default class ShoelaceElement extends LitElement {
       (this.constructor as typeof ShoelaceElement).define(name, component);
     });
   }
+
+  #hasRecordedInitialProperties = false;
+
+  // Store the constructor value of all `static properties = {}`
+  initialReflectedProperties: Map<string, unknown> = new Map();
+
+  attributeChangedCallback(name: string, oldValue: string | null, newValue: string | null) {
+    if (!this.#hasRecordedInitialProperties) {
+      (this.constructor as typeof ShoelaceElement).elementProperties.forEach(
+        (obj, prop: keyof typeof this & string) => {
+          // eslint-disable-next-line
+          if (obj.reflect && this[prop] != null) {
+            this.initialReflectedProperties.set(prop, this[prop]);
+          }
+        }
+      );
+
+      this.#hasRecordedInitialProperties = true;
+    }
+
+    super.attributeChangedCallback(name, oldValue, newValue);
+  }
+
+  protected willUpdate(changedProperties: Parameters<LitElement['willUpdate']>[0]): void {
+    super.willUpdate(changedProperties);
+
+    // Run the morph fixing *after* willUpdate.
+    this.initialReflectedProperties.forEach((value, prop: string & keyof typeof this) => {
+      // If a prop changes to `null`, we assume this happens via an attribute changing to `null`.
+      // eslint-disable-next-line
+      if (changedProperties.has(prop) && this[prop] == null) {
+        // Silly type gymnastics to appease the compiler.
+        (this as Record<string, unknown>)[prop] = value;
+      }
+    });
+  }
 }
 
 export interface ShoelaceFormControl extends ShoelaceElement {