Fix the logic path that merges plain objects

packages/interactivity/src/proxies/state.ts

@@ -332,16 +332,21 @@ const deepMergeRecursive = (
 				}
 			}
 		} else if ( isPlainObject( source[ key ] ) ) {
-			if ( isNew || ( override && ! isPlainObject( target[ key ] ) ) ) {
+			const targetValue = Object.getOwnPropertyDescriptor( target, key )
+				?.value;
+			if ( isNew || ( override && ! isPlainObject( targetValue ) ) ) {
+				// Create a new object if the property is new or needs to be overridden
 				target[ key ] = {};
 				if ( propSignal ) {
 					const ns = getNamespaceFromProxy( proxy );
 					propSignal.setValue(
 						proxifyState( ns, target[ key ] as Object )
 					);
 				}
+				deepMergeRecursive( target[ key ], source[ key ], override );
 			}
-			if ( isPlainObject( target[ key ] ) ) {
+			// Both target and source are plain objects, merge them recursively
+			else if ( isPlainObject( targetValue ) ) {
 				deepMergeRecursive( target[ key ], source[ key ], override );
 			}
 		} else if ( override || isNew ) {

packages/interactivity/src/proxies/test/deep-merge.ts

@@ -473,6 +473,38 @@ describe( 'Interactivity API', () => {
 			expect( target.message.fontStyle ).toBeUndefined();
 		} );
 
+		it( 'should not overwrite getters that become objects if `override` is false', () => {
+			const target: any = proxifyState( 'test', {
+				get message() {
+					return 'hello';
+				},
+			} );
+
+			const getterSpy = jest.spyOn( target, 'message', 'get' );
+
+			let message: any;
+			const spy = jest.fn( () => ( message = target.message ) );
+			effect( spy );
+
+			expect( spy ).toHaveBeenCalledTimes( 1 );
+			expect( message ).toBe( 'hello' );
+
+			deepMerge(
+				target,
+				{ message: { content: 'hello', fontStyle: 'italic' } },
+				false
+			);
+
+			// The effect callback reads `target.message`, so the getter is executed once as well.
+			expect( spy ).toHaveBeenCalledTimes( 1 );
+			expect( getterSpy ).toHaveBeenCalledTimes( 1 );
+
+			expect( message ).toBe( 'hello' );
+			expect( target.message ).toBe( 'hello' );
+			expect( target.message.content ).toBeUndefined();
+			expect( target.message.fontStyle ).toBeUndefined();
+		} );
+
 		it( 'should keep reactivity of arrays that are initially undefined', () => {
 			const target: any = proxifyState( 'test', {} );