Fix animation bug in safari

When showing the sidebar, Safari was causing the sidebar to snap into
place without animating. This is apparently some well-known issue where
it doesn't like adding new elements (or changing display) and toggling
an animated transition in the same event loop.

src/front-end/js/book.js

@@ -540,6 +540,10 @@ aria-label="Show hidden lines"></button>';
         /* To allow the sidebar expansion animation, we first need to put back the display. */
         if (!sidebarCheckbox.checked) {
             sidebar.style.display = '';
+            // Workaround for Safari skipping the animation when changing
+            // `display` and a transform in the same event loop. This forces a
+            // reflow after updating the display.
+            sidebar.offsetHeight;
         }
     });