fix(material/autocomplete): requireSelection incorrectly resetting value when there are no options (#27781)

The autocomplete has a check not to reset the value if the user didn't interact with the input. The problem was that we only accounted for it when there are options, because while technically an autocomplete is _attached_ when the user focuses, we don't consider it _open_ until it shows some options.

Fixes #27767.

(cherry picked from commit db06fa8)

Author: crisbeto

src/material/autocomplete/autocomplete-trigger.ts

@@ -112,8 +112,8 @@ export abstract class _MatAutocompleteTriggerBase
   /** Old value of the native input. Used to work around issues with the `input` event on IE. */
   private _previousValue: string | number | null;
 
-  /** Value of the input element when the panel was opened. */
-  private _valueOnOpen: string | number | null;
+  /** Value of the input element when the panel was attached (even if there are no options). */
+  private _valueOnAttach: string | number | null;
 
   /** Strategy that is used to position the panel. */
   private _positionStrategy: FlexibleConnectedPositionStrategy;
@@ -574,6 +574,7 @@ export abstract class _MatAutocompleteTriggerBase
                 //   of the available options,
                 // - if a valid string is entered after an invalid one.
                 if (this.panelOpen) {
+                  this._captureValueOnAttach();
                   this._emitOpened();
                 } else {
                   this.autocomplete.closed.emit();
@@ -596,10 +597,14 @@ export abstract class _MatAutocompleteTriggerBase
    * the state of the trigger right before the opening sequence was finished.
    */
   private _emitOpened() {
-    this._valueOnOpen = this._element.nativeElement.value;
     this.autocomplete.opened.emit();
   }
 
+  /** Intended to be called when the panel is attached. Captures the current value of the input. */
+  private _captureValueOnAttach() {
+    this._valueOnAttach = this._element.nativeElement.value;
+  }
+
   /** Destroys the autocomplete suggestion panel. */
   private _destroyPanel(): void {
     if (this._overlayRef) {
@@ -650,7 +655,10 @@ export abstract class _MatAutocompleteTriggerBase
       this._onChange(toSelect.value);
       panel._emitSelectEvent(toSelect);
       this._element.nativeElement.focus();
-    } else if (panel.requireSelection && this._element.nativeElement.value !== this._valueOnOpen) {
+    } else if (
+      panel.requireSelection &&
+      this._element.nativeElement.value !== this._valueOnAttach
+    ) {
       this._clearPreviousSelectedOption(null);
       this._assignOptionValue(null);
       // Wait for the animation to finish before clearing the form control value, otherwise
@@ -712,8 +720,8 @@ export abstract class _MatAutocompleteTriggerBase
     this.autocomplete._isOpen = this._overlayAttached = true;
     this.autocomplete._setColor(this._formField?.color);
     this._updatePanelState();
-
     this._applyModalPanelOwnership();
+    this._captureValueOnAttach();
 
     // We need to do an extra `panelOpen` check in here, because the
     // autocomplete won't be shown if there are no options.

src/material/autocomplete/autocomplete.spec.ts

@@ -2663,6 +2663,37 @@ describe('MDC-based MatAutocomplete', () => {
       expect(spy).not.toHaveBeenCalled();
       subscription.unsubscribe();
     }));
+
+    it('should preserve the value if a selection is required, and there are no options', fakeAsync(() => {
+      const input = fixture.nativeElement.querySelector('input');
+      const {stateCtrl, trigger, states} = fixture.componentInstance;
+      fixture.componentInstance.requireSelection = true;
+      stateCtrl.setValue(states[1]);
+      fixture.detectChanges();
+      tick();
+
+      expect(input.value).toBe('California');
+      expect(stateCtrl.value).toEqual({code: 'CA', name: 'California'});
+
+      fixture.componentInstance.states = fixture.componentInstance.filteredStates = [];
+      fixture.detectChanges();
+
+      trigger.openPanel();
+      fixture.detectChanges();
+      zone.simulateZoneExit();
+
+      const spy = jasmine.createSpy('optionSelected spy');
+      const subscription = trigger.optionSelections.subscribe(spy);
+
+      dispatchFakeEvent(document, 'click');
+      fixture.detectChanges();
+      tick();
+
+      expect(input.value).toBe('California');
+      expect(stateCtrl.value).toEqual({code: 'CA', name: 'California'});
+      expect(spy).not.toHaveBeenCalled();
+      subscription.unsubscribe();
+    }));
   });
 
   describe('panel closing', () => {