fix(material/datepicker): do not re-assign the same forms value

src/material/datepicker/datepicker-input-base.ts

@@ -289,22 +289,27 @@ export abstract class MatDatepickerInputBase<S, D = ExtractDateTypeFromSelection
     return this._validator ? this._validator(c) : null;
   }
 
-  // Implemented as part of ControlValueAccessor.
+  /** Implemented as part of ControlValueAccessor. */
   writeValue(value: D): void {
-    this._assignValueProgrammatically(value);
+    // We produce a different date object on each keystroke which can cause signal forms'
+    // interop logic to keep calling `writeValue` with the same value as the user is typing.
+    // Skip such cases since they can prevent the user from typing (see #32442).
+    if (!value || value !== this.value) {
+      this._assignValueProgrammatically(value);
+    }
   }
 
-  // Implemented as part of ControlValueAccessor.
+  /** Implemented as part of ControlValueAccessor. */
   registerOnChange(fn: (value: any) => void): void {
     this._cvaOnChange = fn;
   }
 
-  // Implemented as part of ControlValueAccessor.
+  /** Implemented as part of ControlValueAccessor. */
   registerOnTouched(fn: () => void): void {
     this._onTouched = fn;
   }
 
-  // Implemented as part of ControlValueAccessor.
+  /** Implemented as part of ControlValueAccessor. */
   setDisabledState(isDisabled: boolean): void {
     this.disabled = isDisabled;
   }

src/material/datepicker/datepicker.spec.ts

@@ -1160,6 +1160,22 @@ describe('MatDatepicker', () => {
 
         expect(formControl.hasError('matDatepickerParse')).toBe(true);
       });
+
+      it('should not re-format the input value if the forms module re-assigns the same date', () => {
+        const input = fixture.nativeElement.querySelector('input');
+        const date = new Date(2017, JAN, 1);
+        testComponent.formControl.setValue(date);
+        fixture.detectChanges();
+        expect(input.value).toContain('2017');
+
+        // Note: this isn't how users would behave, but it captures
+        // the sequence of events with signal forms.
+        input.value = 'foo';
+        testComponent.formControl.setValue(date);
+        fixture.detectChanges();
+
+        expect(input.value).toBe('foo');
+      });
     });
 
     describe('datepicker with mat-datepicker-toggle', () => {