Support gallery messages (#6519)

* Support gallery messages

* Address review

* Remove unused function

* Fix indent

* Add "Gallery" message prefix

* Support galleries for image editing

* Fix tapping on a tile opens the last item

* Fix overflow count

* Add caption editing to gallery messages

* Use the new gallery string for prefix

* Lock sending galleries behind a feature flag

* Fix detekts

* Fix

* Ensure image edition is saved when navigating

* Fix sending media broken on edited gallery.

* Fix tests

* Order imports

* Remove unused parameters.

* Fix copyright header of new files.

* Fix Konsist test

* Extract new previews to a dedicated file.

* Sync strings

* Remove unused import

* Update screenshots

* Trigger CI

* Remove parameters with default value.

* More cleanup

* Restore sendAsFile behavior.

* Improve Preview.

* Improve Preview.

* Improve Preview.

* Fix gallery sending cancel and retry issue

* Ensure any previous job is cancelled.

* Fix issue in summary message

* Gallery feature is disabled by default.

* Kotlin convention

* Remove useless parenthesis

* Update screenshots

* Fix test

* List -> ImmutableList

* Remove useless code.

* Render formatted caption for attachment list.

* Replace set of Booleans by an enum

* Remove unused model for individual caption in a gallery Event.

* Fix tests

* Fix tests

* Rework MediaViewer entry point.

And ensure that the clicked image from the gallery is displayed first.

* Ensure gallery item can be click in the pinned message list
Improve the gallery item click handling code.

* Improve code and fix separator color
Closes #7101

* React on attachment item click
Improve code

* Improve code and support 0 items in gallery.

* Fix click on attachment item not rendering anything.

---------

Co-authored-by: Benoit Marty <benoitm@element.io>
Co-authored-by: Benoit Marty <benoit@matrix.org>
Co-authored-by: ElementBot <android@element.io>

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/media/GalleryItemInfo.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.matrix.api.media
+
+import java.io.File
+
+sealed interface GalleryItemInfo {
+    val file: File
+
+    data class Image(
+        override val file: File,
+        val imageInfo: ImageInfo,
+        val thumbnailFile: File?,
+    ) : GalleryItemInfo
+
+    data class Video(
+        override val file: File,
+        val videoInfo: VideoInfo,
+        val thumbnailFile: File?,
+    ) : GalleryItemInfo
+
+    data class Audio(
+        override val file: File,
+        val audioInfo: AudioInfo,
+    ) : GalleryItemInfo
+
+    data class MediaFile(
+        override val file: File,
+        val fileInfo: FileInfo,
+    ) : GalleryItemInfo
+}

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/timeline/Timeline.kt

@@ -16,6 +16,7 @@ import io.element.android.libraries.matrix.api.core.ThreadId
 import io.element.android.libraries.matrix.api.core.TransactionId
 import io.element.android.libraries.matrix.api.media.AudioInfo
 import io.element.android.libraries.matrix.api.media.FileInfo
+import io.element.android.libraries.matrix.api.media.GalleryItemInfo
 import io.element.android.libraries.matrix.api.media.ImageInfo
 import io.element.android.libraries.matrix.api.media.MediaUploadHandler
 import io.element.android.libraries.matrix.api.media.VideoInfo
@@ -157,6 +158,13 @@ interface Timeline : AutoCloseable {
         inReplyToEventId: EventId?,
     ): Result<MediaUploadHandler>
 
+    suspend fun sendGallery(
+        items: List<GalleryItemInfo>,
+        caption: String?,
+        formattedCaption: String?,
+        inReplyToEventId: EventId?,
+    ): Result<MediaUploadHandler>
+
     suspend fun redactEvent(eventOrTransactionId: EventOrTransactionId, reason: String?): Result<Unit>
 
     suspend fun toggleReaction(emoji: String, eventOrTransactionId: EventOrTransactionId): Result<Boolean>

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/timeline/item/event/MessageType.kt

@@ -102,6 +102,21 @@ data class TextMessageType(
     val formatted: FormattedBody?
 ) : MessageType
 
+data class GalleryMessageType(
+    val body: String,
+    val formatted: FormattedBody?,
+    val items: List<GalleryItemType>,
+) : MessageType
+
+@Immutable
+sealed interface GalleryItemType {
+    data class Image(val content: ImageMessageType) : GalleryItemType
+    data class Audio(val content: AudioMessageType) : GalleryItemType
+    data class Video(val content: VideoMessageType) : GalleryItemType
+    data class File(val content: FileMessageType) : GalleryItemType
+    data class Other(val itemType: String, val body: String) : GalleryItemType
+}
+
 data class OtherMessageType(
     val msgType: String,
     val body: String,