Fix reset callback to use resetFallbackValue, if defined. Add tests.

packages/components/src/range-control/index.js

@@ -136,13 +136,29 @@ const BaseRangeControl = forwardRef(
 
 		const handleOnReset = () => {
 			let resetValue = parseFloat( resetFallbackValue );
+			let onChangeResetValue = resetValue;
 
 			if ( isNaN( resetValue ) ) {
 				resetValue = null;
+				onChangeResetValue = undefined;
 			}
 
 			setValue( resetValue );
-			onChange( undefined );
+
+			/**
+			 * Previously, this callback would always receive undefined as
+			 * an argument. This behavior is unexpected, specifically
+			 * when resetFallbackValue is defined.
+			 *
+			 * The value of undefined is not ideal. Passing it through
+			 * to internal <input /> elements would change it from a
+			 * controlled component to an uncontrolled component.
+			 *
+			 * For now, to minimize unexpected regressions, we're going to
+			 * preserve the undefined callback argument, except when a
+			 * resetFallbackValue is defined.
+			 */
+			onChange( onChangeResetValue );
 		};
 
 		const handleShowTooltip = () => setShowTooltip( true );

packages/components/src/range-control/test/index.js

@@ -305,11 +305,13 @@ describe( 'RangeControl', () => {
 
 	describe( 'reset', () => {
 		it( 'should reset to a custom fallback value, defined by a parent component', () => {
+			const spy = jest.fn();
 			const { container } = render(
 				<RangeControl
 					initialPosition={ 50 }
 					value={ 10 }
 					allowReset={ true }
+					onChange={ spy }
 					resetFallbackValue={ 33 }
 				/>
 			);
@@ -322,16 +324,19 @@ describe( 'RangeControl', () => {
 
 			expect( rangeInput.value ).toBe( '33' );
 			expect( numberInput.value ).toBe( '33' );
+			expect( spy ).toHaveBeenCalledWith( 33 );
 		} );
 
 		it( 'should reset to a 50% of min/max value, of no initialPosition or value is defined', () => {
+			const spy = jest.fn();
 			const { container } = render(
 				<RangeControl
 					initialPosition={ undefined }
 					value={ 10 }
 					min={ 0 }
 					max={ 100 }
 					allowReset={ true }
+					onChange={ spy }
 					resetFallbackValue={ undefined }
 				/>
 			);
@@ -344,6 +349,7 @@ describe( 'RangeControl', () => {
 
 			expect( rangeInput.value ).toBe( '50' );
 			expect( numberInput.value ).toBe( '' );
+			expect( spy ).toHaveBeenCalledWith( undefined );
 		} );
 	} );
 } );