feat: improve back-to-top button behavior (#520)

Make the back-to-top button appear based on 
scroll position_ improving accessibility and UX.

src/components/BackToTopButton.astro

@@ -0,0 +1,96 @@
+---
+import IconChevronLeft from "@/assets/icons/IconChevronLeft.svg";
+
+/*
+In #btt-btn-container, left position is `left-[calc((var(--container-3xl)/2)+50%+1rem)]`.
+This is because we use `max-w-3xl` in `PostDetails.astro`.
+If `max-width` in `PostDetails.astro` get updated, make sure to update the `var(--container-3xl)` as well.
+max-w-3xl => var(--container-3xl)
+max-w-4xl => var(--container-4xl)
+max-w-5xl => var(--container-5xl) 
+etc...
+*/
+---
+
+<div
+  id="btt-btn-container"
+  class:list={[
+    "fixed right-4 bottom-8 xl:right-auto xl:left-[calc((var(--container-3xl)/2)+50%+1rem)]",
+    "translate-y-14 opacity-0 transition-all duration-500",
+  ]}
+>
+  <button
+    data-button="back-to-top"
+    class:list={[
+      "group relative",
+      "size-14 rounded-full px-2 py-1 shadow-xl lg:h-8 lg:w-fit lg:rounded-md lg:shadow-none",
+      "bg-background lg:bg-transparent",
+    ]}
+  >
+    <span
+      id="progress-indicator"
+      class="absolute inset-0 -z-10 block size-14 scale-110 rounded-full bg-transparent lg:hidden lg:h-8 lg:rounded-md"
+    ></span>
+    <IconChevronLeft class="inline-block rotate-90 lg:hidden" />
+    <span
+      class="sr-only underline decoration-dashed decoration-2 underline-offset-4 group-hover:text-accent lg:not-sr-only"
+    >
+      <span class="text-xl" tabindex="0">&uarr;</span> Back To Top
+    </span>
+  </button>
+</div>
+
+<script is:inline data-astro-rerun>
+  /** Scrolls the document to the top when
+   * the "Back to Top" button is clicked. */
+  function backToTop() {
+    const rootElement = document.documentElement;
+    const btnContainer = document.querySelector("#btt-btn-container");
+    const backToTopBtn = document.querySelector("[data-button='back-to-top']");
+    const progressIndicator = document.querySelector("#progress-indicator");
+
+    if (!rootElement || !btnContainer || !backToTopBtn || !progressIndicator)
+      return;
+
+    // Attach click event handler for back-to-top button
+    backToTopBtn.addEventListener("click", () => {
+      document.body.scrollTop = 0; // For Safari
+      document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
+    });
+
+    // Handle button visibility according to scroll position
+    let lastVisible = null;
+    function handleScroll() {
+      const scrollTotal = rootElement.scrollHeight - rootElement.clientHeight;
+      const scrollTop = rootElement.scrollTop;
+      const scrollPercent = Math.floor((scrollTop / scrollTotal) * 100);
+
+      progressIndicator.style.setProperty(
+        "background-image",
+        `conic-gradient(var(--accent), var(--accent) ${scrollPercent}%, transparent ${scrollPercent}%)`
+      );
+
+      const isVisible = scrollTop / scrollTotal > 0.3;
+
+      if (isVisible !== lastVisible) {
+        btnContainer.classList.toggle("opacity-100", isVisible);
+        btnContainer.classList.toggle("translate-y-0", isVisible);
+        btnContainer.classList.toggle("opacity-0", !isVisible);
+        btnContainer.classList.toggle("translate-y-14", !isVisible);
+        lastVisible = isVisible;
+      }
+    }
+
+    let ticking = false;
+    document.addEventListener("scroll", () => {
+      if (!ticking) {
+        window.requestAnimationFrame(() => {
+          handleScroll();
+          ticking = false;
+        });
+        ticking = true;
+      }
+    });
+  }
+  backToTop();
+</script>

src/layouts/PostDetails.astro

@@ -8,6 +8,7 @@ import Datetime from "@/components/Datetime.astro";
 import EditPost from "@/components/EditPost.astro";
 import ShareLinks from "@/components/ShareLinks.astro";
 import BackButton from "@/components/BackButton.astro";
+import BackToTopButton from "@/components/BackToTopButton.astro";
 import { getPath } from "@/utils/getPath";
 import { slugifyStr } from "@/utils/slugify";
 import IconChevronLeft from "@/assets/icons/IconChevronLeft.svg";
@@ -109,22 +110,16 @@ const nextPost =
 
     <EditPost class="sm:hidden" {hideEditPost} {post} />
 
-    <ul class="mt-4 mb-8 sm:my-8">
+    <BackToTopButton />
+
+    <div
+      class="flex flex-col items-center justify-between gap-6 sm:flex-row sm:items-start sm:gap-8"
+    >
+      <ul class="self-start">
         {tags.map(tag => <Tag tag={slugifyStr(tag)} tagName={tag} />)}
       </ul>
 
-    <div
-      class="flex flex-col items-center justify-between gap-6 sm:flex-row sm:items-end sm:gap-4"
-    >
       <ShareLinks />
-
-      <button
-        id="back-to-top"
-        class="focus-outline py-1 whitespace-nowrap hover:opacity-75"
-      >
-        <IconChevronLeft class="inline-block rotate-90" />
-        <span>Back to Top</span>
-      </button>
     </div>
 
     <hr class="my-6 border-dashed" />
@@ -270,16 +265,6 @@ const nextPost =
   }
   attachCopyButtons();
 
-  /** Scrolls the document to the top when
-   * the "Back to Top" button is clicked. */
-  function backToTop() {
-    document.querySelector("#back-to-top")?.addEventListener("click", () => {
-      document.body.scrollTop = 0; // For Safari
-      document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
-    });
-  }
-  backToTop();
-
   /* Go to page start after page swap */
   document.addEventListener("astro:after-swap", () =>
     window.scrollTo({ left: 0, top: 0, behavior: "instant" })