feat(media): video thumbnails in feed via VLC first-frame extraction

Add VideoThumbnailCache that grabs the first rendered frame from VLC
and caches it in memory. Videos in the feed now show a thumbnail
preview instead of a blank background before the user clicks play.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/service/media/VideoThumbnailCache.kt

@@ -0,0 +1,129 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.desktop.service.media
+
+import androidx.compose.ui.graphics.ImageBitmap
+import androidx.compose.ui.graphics.toComposeImageBitmap
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import org.jetbrains.skia.Bitmap
+import org.jetbrains.skia.ColorAlphaType
+import org.jetbrains.skia.ImageInfo
+import uk.co.caprica.vlcj.player.base.MediaPlayer
+import uk.co.caprica.vlcj.player.base.MediaPlayerEventAdapter
+import uk.co.caprica.vlcj.player.embedded.videosurface.callback.BufferFormatCallback
+import uk.co.caprica.vlcj.player.embedded.videosurface.callback.RenderCallback
+import uk.co.caprica.vlcj.player.embedded.videosurface.callback.format.RV32BufferFormat
+import java.nio.ByteBuffer
+import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+import org.jetbrains.skia.Image as SkiaImage
+
+object VideoThumbnailCache {
+    private val cache = ConcurrentHashMap<String, ImageBitmap>()
+    private val pending = ConcurrentHashMap<String, Boolean>()
+
+    fun getCached(url: String): ImageBitmap? = cache[url]
+
+    suspend fun getThumbnail(url: String): ImageBitmap? {
+        cache[url]?.let { return it }
+        if (pending.putIfAbsent(url, true) != null) return null
+
+        return withContext(Dispatchers.IO) {
+            try {
+                extractFirstFrame(url)?.also { cache[url] = it }
+            } finally {
+                pending.remove(url)
+            }
+        }
+    }
+
+    private fun extractFirstFrame(url: String): ImageBitmap? {
+        if (!VlcjPlayerPool.init()) return null
+        val player = VlcjPlayerPool.acquire() ?: return null
+
+        var result: ImageBitmap? = null
+        val latch = CountDownLatch(1)
+        var aspectRatio = 16f / 9f
+
+        val bufferFormatCallback =
+            object : BufferFormatCallback {
+                override fun getBufferFormat(
+                    sourceWidth: Int,
+                    sourceHeight: Int,
+                ): uk.co.caprica.vlcj.player.embedded.videosurface.callback.BufferFormat {
+                    if (sourceHeight > 0) {
+                        aspectRatio = sourceWidth.toFloat() / sourceHeight.toFloat()
+                    }
+                    return RV32BufferFormat(sourceWidth, sourceHeight)
+                }
+
+                override fun allocatedBuffers(buffers: Array<out ByteBuffer>) {}
+            }
+
+        val renderCallback =
+            RenderCallback { _, nativeBuffers, bufferFormat ->
+                if (result != null) return@RenderCallback
+                try {
+                    val w = bufferFormat.width
+                    val h = bufferFormat.height
+                    val bmp = Bitmap()
+                    bmp.allocPixels(ImageInfo.makeN32(w, h, ColorAlphaType.PREMUL))
+                    val bytes = ByteArray(w * h * 4)
+                    val buffer = nativeBuffers[0]
+                    buffer.rewind()
+                    buffer.get(bytes)
+                    bmp.installPixels(bytes)
+                    result = SkiaImage.makeFromBitmap(bmp).toComposeImageBitmap()
+                    latch.countDown()
+                } catch (_: Exception) {
+                    latch.countDown()
+                }
+            }
+
+        val surface = VlcjPlayerPool.createVideoSurface(bufferFormatCallback, renderCallback)
+        if (surface == null) {
+            VlcjPlayerPool.release(player)
+            return null
+        }
+
+        player.videoSurface().set(surface)
+        player.audio().setVolume(0)
+        player.audio().isMute = true
+
+        player.events().addMediaPlayerEventListener(
+            object : MediaPlayerEventAdapter() {
+                override fun error(mediaPlayer: MediaPlayer) {
+                    latch.countDown()
+                }
+            },
+        )
+
+        player.media().play(url)
+
+        // Wait up to 5 seconds for first frame
+        latch.await(5, TimeUnit.SECONDS)
+
+        VlcjPlayerPool.release(player)
+        return result
+    }
+}

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/media/DesktopVideoPlayer.kt

@@ -47,6 +47,7 @@ import androidx.compose.ui.graphics.ImageBitmap
 import androidx.compose.ui.graphics.toComposeImageBitmap
 import androidx.compose.ui.layout.ContentScale
 import androidx.compose.ui.unit.dp
+import com.vitorpamplona.amethyst.desktop.service.media.VideoThumbnailCache
 import com.vitorpamplona.amethyst.desktop.service.media.VlcjPlayerPool
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.delay
@@ -72,7 +73,9 @@ fun DesktopVideoPlayer(
     autoPlay: Boolean = false,
     initialSeekPosition: Float = 0f,
     onFullscreen: ((Float) -> Unit)? = null,
-    isFullscreen: Boolean = false,
+    viewMode: ViewMode = ViewMode.DEFAULT,
+    onViewModeChange: ((ViewMode) -> Unit)? = null,
+    trailingControls: @Composable (() -> Unit)? = null,
 ) {
     var frame by remember { mutableStateOf<ImageBitmap?>(null) }
     var isPlaying by remember { mutableStateOf(false) }
@@ -92,6 +95,14 @@ fun DesktopVideoPlayer(
     // Lazy activation — don't touch VLC until user clicks play (or autoPlay)
     var activated by remember { mutableStateOf(autoPlay) }
 
+    // Load thumbnail when not activated
+    var thumbnail by remember(url) { mutableStateOf(VideoThumbnailCache.getCached(url)) }
+    LaunchedEffect(url, activated) {
+        if (!activated && thumbnail == null) {
+            thumbnail = VideoThumbnailCache.getThumbnail(url)
+        }
+    }
+
     // Pause when another player becomes active
     val activeId by ActiveMediaManager.activeId.collectAsState()
     LaunchedEffect(activeId) {
@@ -248,7 +259,8 @@ fun DesktopVideoPlayer(
                 ),
         contentAlignment = Alignment.Center,
     ) {
-        frame?.let { bitmap ->
+        val displayBitmap = frame ?: thumbnail
+        displayBitmap?.let { bitmap ->
             Image(
                 bitmap = bitmap,
                 contentDescription = "Video",
@@ -268,7 +280,7 @@ fun DesktopVideoPlayer(
             currentTime = currentTime,
             volume = volume,
             isMuted = isMuted,
-            isFullscreen = isFullscreen,
+            viewMode = viewMode,
             onPlayPause = {
                 val p = player
                 if (p != null) {
@@ -305,6 +317,8 @@ fun DesktopVideoPlayer(
                 } else {
                     null
                 },
+            onViewModeChange = onViewModeChange,
+            trailingControls = trailingControls,
         )
     }
 }