fix(DropdownMenu): Allow positionFixed prop to be passed through

Allow the positionFixed prop to be passed through to the Popper component

Fixes reactstrap#1473

docs/lib/Components/DropdownsPage.js

@@ -94,7 +94,9 @@ DropdownMenu.propTypes = {
   cssModule: PropTypes.object,
   // Custom modifiers that are passed to DropdownMenu.js, see https://popper.js.org/popper-documentation.html#modifiers
   modifiers: PropTypes.object,
-  persist: PropTypes.bool // presist the popper, even when closed. See #779 for reasoning
+  persist: PropTypes.bool, // presist the popper, even when closed. See #779 for reasoning
+  // passed to popper, see https://popper.js.org/popper-documentation.html#Popper.Defaults.positionFixed
+  positionFixed: PropTypes.bool
 };
 
 DropdownItem.propTypes = {

src/DropdownMenu.js

@@ -14,6 +14,7 @@ const propTypes = {
   className: PropTypes.string,
   cssModule: PropTypes.object,
   persist: PropTypes.bool,
+  positionFixed: PropTypes.bool,
 };
 
 const defaultProps = {
@@ -33,7 +34,7 @@ const directionPositionMap = {
 class DropdownMenu extends React.Component { 
 
   render() {
-    const { className, cssModule, right, tag, flip, modifiers, persist, ...attrs } = this.props;
+    const { className, cssModule, right, tag, flip, modifiers, persist, positionFixed, ...attrs } = this.props;
     const classes = mapToCssModules(classNames(
       className,
       'dropdown-menu',
@@ -54,11 +55,13 @@ class DropdownMenu extends React.Component {
         ...modifiers,
         ...noFlipModifier,
       } : modifiers;
+      const poperPositionFixed = !!positionFixed;
 
       return (
         <Popper
           placement={poperPlacement}
           modifiers={poperModifiers}
+          positionFixed={poperPositionFixed}
         >
           {({ ref, style, placement }) => (
             <Tag

src/__tests__/DropdownMenu.spec.js

@@ -160,6 +160,17 @@ describe('DropdownMenu', () => {
     expect(wrapper.find(Popper).prop('modifiers')).toEqual({ flip: { enabled: false } });
   });
 
+  it('should position using fixed mode when positionFixed is true', () => {
+    isOpen = true;
+    const wrapper = mount(
+      <DropdownContext.Provider value={{ isOpen, direction, inNavbar }}>
+        <DropdownMenu positionFixed>Ello world</DropdownMenu>
+      </DropdownContext.Provider>
+    );
+
+    expect(wrapper.find(Popper).prop('positionFixed')).toBe(true);
+  });
+
   it('should not render multiple children when isOpen is false', () => {
     const wrapper = mount(
       <DropdownContext.Provider value={{ isOpen, direction, inNavbar }}>