Only treat animated gifs as videos, leave static gifs as images (#9814)

* only treat *animated* gifs as videos

* fix export

src/view/com/composer/videos/isAnimatedGif.ts

@@ -0,0 +1,59 @@
+/**
+ * Checks if a GIF is animated. Cooked up by Claude, validated with some examples.
+ * @param bytes - The GIF bytes, as a Uint8Array.
+ * @returns An object with properties isGif, isAnimated, and frames.
+ */
+export function isAnimatedGif(buffer: ArrayBuffer): {
+  isGif: boolean
+  isAnimated: boolean
+  frames: number
+} {
+  const bytes = new Uint8Array(buffer)
+  // Verify GIF signature
+  const sig = String.fromCharCode(...bytes.slice(0, 6))
+  if (!sig.startsWith('GIF'))
+    return {isGif: false, isAnimated: false, frames: 0}
+
+  let i = 13 // Skip header + logical screen descriptor
+
+  // Skip global color table if present
+  if (bytes[10] & 0x80) {
+    const gctSize = 3 * (1 << ((bytes[10] & 0x07) + 1))
+    i += gctSize
+  }
+
+  let frames = 0
+
+  while (i < bytes.length) {
+    const block = bytes[i++]
+
+    if (block === 0x2c) {
+      // Image descriptor
+      frames++
+
+      // Skip image descriptor fields
+      i += 8
+      // Skip local color table if present
+      if (bytes[i] & 0x80) {
+        const lctSize = 3 * (1 << ((bytes[i] & 0x07) + 1))
+        i += lctSize + 1
+      } else {
+        i++
+      }
+      // Skip image data blocks
+      i++ // LZW minimum code size
+      while (bytes[i]) i += bytes[i] + 1
+      i++
+    } else if (block === 0x21) {
+      // Extension
+      i++ // Extension type
+      while (bytes[i]) i += bytes[i] + 1
+      i++
+    } else if (block === 0x3b) {
+      // Trailer
+      break
+    }
+  }
+
+  return {isGif: true, isAnimated: frames > 1, frames}
+}