fix(combobox): improve prop update times (#11383)

**Related Issue:** #10731 

## Summary

Fixes prop update delays introduced in #10310 by moving event emitting
to Lit's [pre-update
phase](https://lit.dev/docs/components/lifecycle/#reactive-update-cycle).

packages/calcite-components/src/components/combobox-item/combobox-item.tsx

@@ -41,6 +41,12 @@ export class ComboboxItem extends LitElement implements InteractiveComponent {
 
   // #endregion
 
+  // #region Private Properties
+
+  private _selected = false;
+
+  // #endregion
+
   // #region Public Properties
 
   /** When `true`, the component is active. */
@@ -91,7 +97,18 @@ export class ComboboxItem extends LitElement implements InteractiveComponent {
   @property() scale: Scale = "m";
 
   /** When `true`, the component is selected. */
-  @property({ reflect: true }) selected: boolean = false;
+  @property({ reflect: true })
+  get selected(): boolean {
+    return this._selected;
+  }
+  set selected(value: boolean) {
+    const oldValue = this._selected;
+    if (value !== oldValue) {
+      this._selected = value;
+      // we emit directly to avoid delays updating the parent combobox
+      this.emitItemChange();
+    }
+  }
 
   /**
    * Specifies the selection mode of the component, where:
@@ -168,18 +185,14 @@ export class ComboboxItem extends LitElement implements InteractiveComponent {
   }
 
   override willUpdate(changes: PropertyValues<this>): void {
-    /* TODO: [MIGRATION] First time Lit calls willUpdate(), changes will include not just properties provided by the user, but also any default values your component set.
-    To account for this semantics change, the checks for (this.hasUpdated || value != defaultValue) was added in this method
-    Please refactor your code to reduce the need for this check.
-    Docs: https://qawebgis.esri.com/arcgis-components/?path=/docs/lumina-transition-from-stencil--docs#watching-for-property-changes */
     if (
-      (changes.has("disabled") && this.hasUpdated) ||
-      (changes.has("selected") && this.hasUpdated) ||
-      (changes.has("textLabel") && this.hasUpdated) ||
-      (changes.has("heading") && this.hasUpdated) ||
-      (changes.has("label") && this.hasUpdated)
+      this.hasUpdated &&
+      (changes.has("disabled") ||
+        changes.has("heading") ||
+        changes.has("label") ||
+        changes.has("textLabel"))
     ) {
-      this.calciteInternalComboboxItemChange.emit();
+      this.emitItemChange();
     }
   }
 
@@ -191,6 +204,10 @@ export class ComboboxItem extends LitElement implements InteractiveComponent {
 
   // #region Private Methods
 
+  private emitItemChange(): void {
+    this.calciteInternalComboboxItemChange.emit();
+  }
+
   private handleDefaultSlotChange(event: Event): void {
     this.hasContent = slotChangeHasContent(event);
   }

packages/calcite-components/src/components/combobox/combobox.e2e.ts

@@ -1363,6 +1363,26 @@ describe("calcite-combobox", () => {
       expect(await item2.getProperty("selected")).toBe(true);
       expect(eventSpy).toHaveReceivedEventTimes(1);
     });
+
+    it("updates the value immediately after selecting an item programmatically", async () => {
+      const page = await newE2EPage();
+      await page.setContent(html`
+        <calcite-combobox selection-mode="single">
+          <calcite-combobox-item value="1" text-label="first"></calcite-combobox-item>
+          <calcite-combobox-item value="2" text-label="second"></calcite-combobox-item>
+          <calcite-combobox-item value="3" text-label="third"></calcite-combobox-item>
+        </calcite-combobox>
+      `);
+
+      const immediateValueAfterSelected = await page.evaluate(async () => {
+        const combobox = document.querySelector("calcite-combobox");
+        const firstItem = document.querySelector("calcite-combobox-item");
+        firstItem.selected = true;
+        return combobox.value;
+      });
+
+      expect(immediateValueAfterSelected).toBe("1");
+    });
   });
 
   describe("clearing values", () => {

packages/calcite-components/src/components/combobox/combobox.tsx

@@ -252,6 +252,8 @@ export class Combobox
 
   private selectedIndicatorChipEl: Chip["el"];
 
+  private _selectedItems: HTMLCalciteComboboxItemElement["el"][] = [];
+
   private get showingInlineIcon(): boolean {
     const { placeholderIcon, selectionMode, selectedItems, open } = this;
     const selectedItem = selectedItems[0];
@@ -397,7 +399,17 @@ export class Combobox
    *
    * @readonly
    */
-  @property() selectedItems: HTMLCalciteComboboxItemElement["el"][] = [];
+  @property() get selectedItems(): HTMLCalciteComboboxItemElement["el"][] {
+    return this._selectedItems;
+  }
+
+  set selectedItems(selectedItems: HTMLCalciteComboboxItemElement["el"][]) {
+    const oldSelectedItems = this._selectedItems;
+    if (selectedItems !== oldSelectedItems) {
+      this._selectedItems = selectedItems;
+      this.selectedItemsHandler();
+    }
+  }
 
   /**
    * When `selectionMode` is `"ancestors"` or `"multiple"`, specifies the display of multiple `calcite-combobox-item` selections, where:
@@ -594,10 +606,6 @@ export class Combobox
     if (changes.has("flipPlacements")) {
       this.flipPlacementsHandler();
     }
-
-    if (changes.has("selectedItems") && (this.hasUpdated || this.selectedItems?.length > 0)) {
-      this.selectedItemsHandler();
-    }
   }
 
   override updated(): void {
@@ -741,8 +749,8 @@ export class Combobox
   }
 
   private getValue(): string | string[] {
-    const items = this.selectedItems.map((item) => item?.value?.toString());
-    return items?.length ? (items.length > 1 ? items : items[0]) : "";
+    const items = this.selectedItems.map((item) => item.value?.toString());
+    return items.length ? (items.length > 1 ? items : items[0]) : "";
   }
 
   private comboboxInViewport(): boolean {