Prefetch fresh video thumbnail before swapping it into info view

Yattee/Services/ImageLoading/ImageLoadingService.swift

@@ -81,6 +81,22 @@ final class ImageLoadingService: Sendable {
         )
     }
 
+    /// Warm the image cache for the given URL so a subsequent `LazyImage`
+    /// display hits the cache instantly. Returns after the image is cached
+    /// or the operation fails (silently). Bounded by `timeout` seconds.
+    nonisolated func prefetchImage(for url: URL, timeout: TimeInterval = 3) async {
+        await withTaskGroup(of: Void.self) { group in
+            group.addTask {
+                _ = try? await ImagePipeline.shared.image(for: url)
+            }
+            group.addTask {
+                try? await Task.sleep(nanoseconds: UInt64(timeout * 1_000_000_000))
+            }
+            await group.next()
+            group.cancelAll()
+        }
+    }
+
     /// Remove a specific URL from both the memory and disk image caches.
     /// Use when a previously cached URL is known to return stale or broken
     /// data (e.g. an expired proxied thumbnail URL).

Yattee/Views/Video/VideoInfoView.swift

@@ -2165,6 +2165,7 @@ struct VideoInfoView: View {
             do {
                 // Use extractURL method - just use the video part
                 let (fullVideo, _, _) = try await contentService.extractURL(originalURL, instance: instance)
+                await prefetchNewThumbnailIfNeeded(old: base, new: fullVideo)
                 invalidateStaleThumbnails(old: base, new: fullVideo)
                 loadedVideoDetails[videoID] = fullVideo
                 CachedChannelData.cacheAuthor(fullVideo.author)
@@ -2190,6 +2191,7 @@ struct VideoInfoView: View {
                 id: videoID,
                 instance: instance
             )
+            await prefetchNewThumbnailIfNeeded(old: base, new: fullVideo)
             invalidateStaleThumbnails(old: base, new: fullVideo)
             loadedVideoDetails[videoID] = fullVideo
             CachedChannelData.cacheAuthor(fullVideo.author)
@@ -2200,6 +2202,20 @@ struct VideoInfoView: View {
         isLoadingVideoDetails = false
     }
 
+    /// Warms the Nuke cache for the freshly fetched thumbnail *before* we
+    /// swap it into the view. Without this, the view observes a URL change,
+    /// flips `LazyImage` to the new URL, and briefly shows a placeholder
+    /// while the download happens — the "thumbnail flash". By the time the
+    /// swap lands, the image is already cached.
+    @MainActor
+    private func prefetchNewThumbnailIfNeeded(old: Video, new: Video) async {
+        guard let newURL = new.bestThumbnail?.url else { return }
+        let oldKey = old.bestThumbnail.map { ImageLoadingService.cacheKey(for: $0.url) }
+        let newKey = ImageLoadingService.cacheKey(for: newURL)
+        guard oldKey != newKey else { return }
+        await ImageLoadingService.shared.prefetchImage(for: newURL)
+    }
+
     /// Evicts image cache entries for thumbnails whose URLs changed between
     /// the queued/list copy of the video and the freshly fetched details, so
     /// other views holding the old URLs re-fetch instead of reusing broken