Fix pricing page scrolling

A scrolling/focus issue was noted in #167. The error was unconditionally
scrolling to the pricing plan blocks. The logic that we want is to only
scroll to the plans if they are not on the page currently, and only when
some action has selected one of the tabs (there is a URL hash).

src/js/site.js

@@ -207,7 +207,13 @@ jquery.fn.plausible = function () {
 jquery.fn.tabmenu = function (settings) {
   return this.each((index, elem) => {
     settings.onLoad = () => {
-      $(elem).get(0).scrollIntoView();
+      // Only scroll to the plan block when it is not on screen. We also inspect
+      // the URL hash to ensure we're not scrolling on navigating to the page.
+      // If there is a page hash, something was clicked to select the plans tab.
+      const is_on_screen = $(elem).visibility("is on screen");
+      if (!is_on_screen && window.location.hash) {
+        $(elem).get(0).scrollIntoView();
+      }
     };
     $(elem).find(".item").tab(settings);
   });