Add support for displaying link previews for Admin UI (#35958)

app/javascript/entrypoints/admin.tsx

@@ -1,6 +1,7 @@
 import { createRoot } from 'react-dom/client';
 
 import Rails from '@rails/ujs';
+import { decode, ValidationError } from 'blurhash';
 
 import ready from '../mastodon/ready';
 
@@ -362,6 +363,46 @@ ready(() => {
   document.querySelectorAll('[data-admin-component]').forEach((element) => {
     void mountReactComponent(element);
   });
+
+  document
+    .querySelectorAll<HTMLCanvasElement>('canvas[data-blurhash]')
+    .forEach((canvas) => {
+      const blurhash = canvas.dataset.blurhash;
+      if (blurhash) {
+        try {
+          // decode returns a Uint8ClampedArray<ArrayBufferLike> not Uint8ClampedArray<ArrayBuffer>
+          const pixels = decode(
+            blurhash,
+            32,
+            32,
+          ) as Uint8ClampedArray<ArrayBuffer>;
+          const ctx = canvas.getContext('2d');
+          const imageData = new ImageData(pixels, 32, 32);
+
+          ctx?.putImageData(imageData, 0, 0);
+        } catch (err) {
+          if (err instanceof ValidationError) {
+            // ignore blurhash validation errors
+            return;
+          }
+
+          throw err;
+        }
+      }
+    });
+
+  document
+    .querySelectorAll<HTMLDivElement>('.preview-card')
+    .forEach((previewCard) => {
+      const spoilerButton = previewCard.querySelector('.spoiler-button');
+      if (!spoilerButton) {
+        return;
+      }
+
+      spoilerButton.addEventListener('click', () => {
+        previewCard.classList.toggle('preview-card--image-visible');
+      });
+    });
 }).catch((reason: unknown) => {
   throw reason;
 });

app/javascript/styles/mastodon/admin.scss

@@ -1969,6 +1969,61 @@ a.sparkline {
       display: list-item;
     }
   }
+
+  .preview-card {
+    position: relative;
+    max-width: 566px;
+
+    .status-card__image {
+      &--video {
+        aspect-ratio: 16 / 9;
+      }
+
+      &--large {
+        aspect-ratio: 1.91 / 1;
+      }
+
+      aspect-ratio: 1;
+    }
+
+    .spoiler-button__overlay__label {
+      outline: 1px solid var(--media-outline-color);
+    }
+
+    .hide-button {
+      // Toggled to appear when the preview-card is unblurred:
+      display: none;
+      position: absolute;
+      top: 5px;
+      right: 5px;
+      color: $white;
+      border: 0;
+      outline: 1px solid var(--media-outline-color);
+      background-color: color.change($black, $alpha: 0.45);
+      backdrop-filter: $backdrop-blur-filter;
+      padding: 3px 12px;
+      border-radius: 99px;
+      font-size: 14px;
+      font-weight: 700;
+      line-height: 20px;
+
+      &:hover,
+      &:focus {
+        background-color: color.change($black, $alpha: 0.9);
+      }
+    }
+
+    &.preview-card--image-visible {
+      .hide-button {
+        display: block;
+      }
+
+      .spoiler-button__overlay,
+      .status-card__image-preview {
+        display: none;
+      }
+    }
+  }
 }
 
 .admin {