Address copilot review

- Remove back-to-top from tab order while hidden
- Add visibility:hidden alongside opacity/pointer-events so keyboard and
  screen-reader users can't focus the button when it's not shown.
- Replace inline style with a semantic .sponsor-cta class
- Update footer copyright year range to 2026
- Pre-bake hero background blur/saturation, drop CSS filter
- Replace the runtime CSS `filter: blur(1px) saturate(1.3)` on the
  full-viewport .hero::before layer with a pre-processed JPEG
  (blurred/saturated at build time via Pillow), avoiding the GPU compositing
  cost of filtering a large layer on every paint. Visual result is unchanged.
- Fix grammar in sponsor pitch: hyphenate well-maintained, add comma
- Name back-to-top bottom-fudge constant for clarity

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclark4life

assets/css/pillow.css

@@ -5,10 +5,12 @@
    - Coral/terracotta and magenta are pulled directly from the flowers in
      the Pillow logo photo.
    - Sky blue is pulled from the desert/sky photo in the same logo.
-   - All three also appear in assets/images/psychedelic-glow.png, the
+   - All three also appear in assets/images/psychedelic-glow.jpg, the
      psychedelic rainbow texture from the original PIL-era site (art by
      Jeremy Kun), which we reuse as a subtle glow behind the hero logo for
-     a bit of the site's history and visual uniqueness.
+     a bit of the site's history and visual uniqueness. Pre-blurred and
+     pre-saturated at build time (rather than via CSS `filter`) to avoid
+     the runtime GPU cost of filtering a full-viewport layer.
    - Ink (near-black) and warm paper (off-white) anchor everything so the
      brighter accent colors stay legible and don't overwhelm the page.
    ========================================================================== */
@@ -239,14 +241,15 @@ img {
 }
 
 .hero::before {
-    /* Psychedelic art by Jeremy Kun, from the original PIL-era site */
+    /* Psychedelic art by Jeremy Kun, from the original PIL-era site.
+       Blur/saturation are pre-baked into the image itself (see the
+       palette rationale note above), so no CSS filter is needed here. */
     content: "";
     position: absolute;
     inset: 0;
-    background-image: url(../images/psychedelic-glow.png);
+    background-image: url(../images/psychedelic-glow.jpg);
     background-size: cover;
     background-position: center;
-    filter: blur(1px) saturate(1.3);
     opacity: .45;
     mix-blend-mode: screen;
     z-index: -2;
@@ -469,6 +472,10 @@ section {
     margin-right: .5rem;
 }
 
+.sponsor-cta {
+    text-align: center;
+}
+
 @media (max-width: 760px) {
     .sponsor-grid {
         grid-template-columns: 1fr;
@@ -737,13 +744,15 @@ section {
     font-size: 1.1rem;
     z-index: 10;
     opacity: 0;
+    visibility: hidden;
     transform: translateY(.5rem);
     pointer-events: none;
-    transition: opacity .2s ease-in-out, transform .2s ease-in-out, background-color .15s ease-in-out;
+    transition: opacity .2s ease-in-out, transform .2s ease-in-out, background-color .15s ease-in-out, visibility .2s;
 }
 
 .back-to-top.is-visible {
     opacity: 1;
+    visibility: visible;
     transform: translateY(0);
     pointer-events: auto;
 }

assets/images/psychedelic-glow.jpg

@@ -5,13 +5,16 @@
     if (!button) return;
 
     var SHOW_AFTER_PX = 600;
+    // Small fudge factor so sub-pixel scroll rounding still counts as "at
+    // the bottom" of the page.
+    var BOTTOM_FUDGE_PX = 4;
     var ticking = false;
 
     function update() {
         var scrollY = window.scrollY || window.pageYOffset;
         var pageHeight = document.documentElement.scrollHeight;
         var isScrollable = pageHeight > window.innerHeight;
-        var atBottom = isScrollable && window.innerHeight + scrollY >= pageHeight - 4;
+        var atBottom = isScrollable && window.innerHeight + scrollY >= pageHeight - BOTTOM_FUDGE_PX;
         button.classList.toggle("is-visible", scrollY > SHOW_AFTER_PX || atBottom);
         ticking = false;
     }

assets/images/psychedelic-glow.png

@@ -119,7 +119,7 @@ <h2><i class="fas fa-heart" aria-hidden="true"></i> Support Pillow's Future</h2>
                             Pillow is free, open source software maintained by volunteers who review pull requests,
                             fix bugs, write documentation, and ship regular releases &mdash; all in their spare time.
                             Millions of projects depend on Pillow every day. If Pillow powers your product or
-                            business, consider sponsoring us to help keep it secure, fast and well maintained
+                            business, consider sponsoring us to help keep it secure, fast, and well-maintained
                             for everyone.
                         </p>
                     </div>
@@ -128,7 +128,7 @@ <h2><i class="fas fa-heart" aria-hidden="true"></i> Support Pillow's Future</h2>
                         <div class="sponsor-chip"><i class="fas fa-code-branch" aria-hidden="true"></i> Prioritize reviewing PRs and issues</div>
                         <div class="sponsor-chip"><i class="fas fa-users" aria-hidden="true"></i> Sustain the maintainers who ship Pillow</div>
                     </div>
-                    <div style="text-align: center;">
+                    <div class="sponsor-cta">
                         <a class="btn btn-sponsor-light btn-lg" href="https://github.com/sponsors/python-pillow">
                             <i class="fas fa-heart" aria-hidden="true"></i> Become a GitHub Sponsor</a>
                     </div>
@@ -296,7 +296,7 @@ <h4><i class="fas fa-heart" aria-hidden="true"></i> Support Pillow</h4>
             </div>
             <div class="wrap footer-bottom">
                 <p>
-                    &copy; 1995-2011 Fredrik Lundh and contributors. &copy; 2010-2024 Jeffrey 'Alex' Clark and
+                    &copy; 1995-2011 Fredrik Lundh and contributors. &copy; 2010-2026 Jeffrey 'Alex' Clark and
                     <a href="https://github.com/python-pillow/Pillow/graphs/contributors">contributors</a>.
                     Logo by Alastair Houghton. Psychedelic art by Jeremy Kun.
                 </p>