fix(gatsby-react-router-scroll): Respect hash as source of truth for scroll position (#28555)

(cherry picked from commit a066a71)

Author: sidharthachatterjee

e2e-tests/production-runtime/cypress/integration/scroll-behavior.js

@@ -48,7 +48,7 @@ describe(`Scroll behaviour`, () => {
       cy.go(`forward`).waitForRouteChange()
 
       cy.window().then(updatedWindow => {
-        expect(updatedWindow.scrollY).not.to.eq(idScrollY)
+        expect(updatedWindow.scrollY).to.eq(idScrollY)
       })
     })
   })

packages/gatsby-react-router-scroll/src/scroll-handler.tsx

@@ -61,7 +61,16 @@ export class ScrollHandler extends React.Component<
       scrollPosition = this._stateStorage.read(this.props.location, key)
     }
 
-    if (hash && scrollPosition === 0) {
+    /**  There are two pieces of state: the browser url and
+     * history state which keeps track of scroll position
+     * Native behaviour prescribes that we ought to restore scroll position
+     * when a user navigates back in their browser (this is the `POP` action)
+     * Currently, reach router has a bug that prevents this at https://github.com/reach/router/issues/228
+     * So we _always_ stick to the url as a source of truth — if the url
+     * contains a hash, we scroll to it
+     */
+
+    if (hash) {
       this.scrollToHash(decodeURI(hash), prevProps)
     } else {
       this.windowScroll(scrollPosition, prevProps)