diff --git a/.idea/inspectionProfiles/ktlint.xml b/.idea/inspectionProfiles/ktlint.xml
index 4aab2f7215ef..631c09bc2163 100644
--- a/.idea/inspectionProfiles/ktlint.xml
+++ b/.idea/inspectionProfiles/ktlint.xml
@@ -33,6 +33,7 @@
       <option name="composableFile" value="true" />
     </inspection_tool>
     <inspection_tool class="KotlinUnusedImport" enabled="true" level="ERROR" enabled_by_default="true" />
+    <inspection_tool class="LongLine" enabled="true" level="WARNING" enabled_by_default="true" />
     <inspection_tool class="PreviewAnnotationInFunctionWithParameters" enabled="true" level="ERROR" enabled_by_default="true">
       <option name="composableFile" value="true" />
       <option name="previewFile" value="true" />
diff --git a/app/src/androidTest/java/com/owncloud/android/ui/fragment/GalleryFragmentIT.kt b/app/src/androidTest/java/com/owncloud/android/ui/fragment/GalleryFragmentIT.kt
index 0bdf133e055f..3b4ede5f274b 100644
--- a/app/src/androidTest/java/com/owncloud/android/ui/fragment/GalleryFragmentIT.kt
+++ b/app/src/androidTest/java/com/owncloud/android/ui/fragment/GalleryFragmentIT.kt
@@ -1,6 +1,7 @@
 /*
  * Nextcloud - Android Client
  *
+ * SPDX-FileCopyrightText: 2026 Philipp Hasper <vcs@hasper.info>
  * SPDX-FileCopyrightText: 2025 Alper Ozturk <alper.ozturk@nextcloud.com>
  * SPDX-FileCopyrightText: 2022 Tobias Kaminsky <tobias@kaminsky.me>
  * SPDX-FileCopyrightText: 2022 Nextcloud GmbH
@@ -12,24 +13,37 @@ import android.graphics.Bitmap
 import android.graphics.Canvas
 import android.graphics.Color
 import android.graphics.Paint
+import android.view.View
+import android.view.ViewGroup
+import android.widget.FrameLayout
+import androidx.recyclerview.widget.RecyclerView
 import androidx.test.core.app.launchActivity
 import androidx.test.espresso.Espresso.onView
+import androidx.test.espresso.UiController
+import androidx.test.espresso.ViewAction
 import androidx.test.espresso.assertion.ViewAssertions.matches
+import androidx.test.espresso.contrib.RecyclerViewActions
+import androidx.test.espresso.contrib.RecyclerViewActions.actionOnItemAtPosition
 import androidx.test.espresso.matcher.ViewMatchers.isDisplayed
 import androidx.test.espresso.matcher.ViewMatchers.isRoot
+import androidx.test.espresso.matcher.ViewMatchers.withId
 import com.nextcloud.test.TestActivity
 import com.owncloud.android.AbstractIT
+import com.owncloud.android.R
 import com.owncloud.android.datamodel.OCFile
 import com.owncloud.android.datamodel.ThumbnailsCacheManager
 import com.owncloud.android.datamodel.ThumbnailsCacheManager.PREFIX_RESIZED_IMAGE
 import com.owncloud.android.lib.common.utils.Log_OC
 import com.owncloud.android.lib.resources.files.model.ImageDimension
+import com.owncloud.android.ui.adapter.GalleryRowHolder
 import com.owncloud.android.utils.ScreenshotTest
 import org.junit.After
+import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNotNull
 import org.junit.Before
 import org.junit.Test
 import java.util.Random
+import org.hamcrest.Matchers.`is` as isSameView
 
 class GalleryFragmentIT : AbstractIT() {
     private val testClassName = "com.owncloud.android.ui.fragment.GalleryFragmentIT"
@@ -85,14 +99,99 @@ class GalleryFragmentIT : AbstractIT() {
         }
     }
 
-    private fun createImage(id: Int, width: Int? = null, height: Int? = null) {
+    @Test
+    fun multiSelect() {
+        val imageCount = 100
+        for (num in 1..imageCount) {
+            // Spread the files over multiple days to also get multiple sections
+            val secondsPerDay = 1L * 24 * 60 * 60
+            createImage(10000000 + num * 7 * secondsPerDay, 700, 300)
+        }
+
+        // Test that scrolling through the whole list is possible without a crash
+        launchActivity<TestActivity>().use { scenario ->
+            lateinit var galleryFragment: GalleryFragment
+            scenario.onActivity { testActivity ->
+                galleryFragment = GalleryFragment()
+                testActivity.addFragment(galleryFragment)
+            }
+            onView(isRoot()).check(matches(isDisplayed()))
+
+            onView(withId(R.id.list_root))
+                .perform(RecyclerViewActions.scrollToLastPosition<GalleryRowHolder>())
+                .perform(RecyclerViewActions.scrollToPosition<GalleryRowHolder>(0))
+        }
+
+        // Test selection of all entries
+        launchActivity<TestActivity>().use { scenario ->
+            lateinit var galleryFragment: GalleryFragment
+            scenario.onActivity { testActivity ->
+                galleryFragment = GalleryFragment()
+                testActivity.addFragment(galleryFragment)
+            }
+            onView(isRoot()).check(matches(isDisplayed()))
+
+            // get the RecyclerView and itemCount on the UI thread
+            val recyclerView = findRecyclerViewRecursively(galleryFragment.view)
+                ?: throw AssertionError("RecyclerView not found")
+            val adapterCount = recyclerView.adapter?.itemCount ?: 0
+
+            // Perform the view action on each adapter position (row)
+            for (pos in 0 until adapterCount) {
+                onView(isSameView(recyclerView))
+                    .perform(actionOnItemAtPosition<RecyclerView.ViewHolder>(pos, longClickAllThumbnailsInRow()))
+            }
+
+            val checked = galleryFragment.commonAdapter.getCheckedItems()
+            assertEquals(imageCount, checked.size)
+        }
+    }
+
+    /** Recursively walk view tree to find the first RecyclerView. Runs on the same thread that calls it. */
+    @Suppress("ReturnCount")
+    private fun findRecyclerViewRecursively(root: View?): RecyclerView? {
+        if (root == null) return null
+        if (root is RecyclerView) return root
+        if (root !is ViewGroup) return null
+        for (i in 0 until root.childCount) {
+            val child = root.getChildAt(i)
+            val found = findRecyclerViewRecursively(child)
+            if (found != null) return found
+        }
+        return null
+    }
+
+    /** For the given row view, long-click each thumbnail inside its FrameLayouts */
+    @Suppress("NestedBlockDepth")
+    fun longClickAllThumbnailsInRow() = object : ViewAction {
+        override fun getConstraints() = isDisplayed()
+
+        override fun getDescription() = "Long-click all thumbnail ImageViews inside a GalleryRowHolder"
+
+        override fun perform(uiController: UiController, view: View) {
+            if (view is ViewGroup) {
+                // each child of the row is a FrameLayout representing one gallery cell
+                for (i in 0 until view.childCount) {
+                    val cell = view.getChildAt(i)
+                    if (cell is FrameLayout) {
+                        // GalleryRowHolder builds FrameLayout with children:
+                        // 0 = shimmer, 1 = thumbnail ImageView, 2 = checkbox
+                        val thumbnail = if (cell.childCount > 1) cell.getChildAt(1) else cell
+                        thumbnail.performLongClick()
+                    }
+                }
+            }
+        }
+    }
+
+    private fun createImage(id: Long, width: Int? = null, height: Int? = null) {
         val defaultSize = ThumbnailsCacheManager.getThumbnailDimension().toFloat()
         val file = OCFile("/$id.png").apply {
-            fileId = id.toLong()
+            fileId = id
             remoteId = "$id"
             mimeType = "image/png"
             isPreviewAvailable = true
-            modificationTimestamp = (1658475504 + id.toLong()) * 1000
+            modificationTimestamp = (1658475504 + id) * 1000
             imageDimension = ImageDimension(width?.toFloat() ?: defaultSize, height?.toFloat() ?: defaultSize)
             storageManager.saveFile(this)
         }
diff --git a/app/src/main/java/com/owncloud/android/datamodel/GalleryRow.kt b/app/src/main/java/com/owncloud/android/datamodel/GalleryRow.kt
index febd3aaf19a4..6e99de382fae 100644
--- a/app/src/main/java/com/owncloud/android/datamodel/GalleryRow.kt
+++ b/app/src/main/java/com/owncloud/android/datamodel/GalleryRow.kt
@@ -14,5 +14,4 @@ data class GalleryRow(val files: List<OCFile>, val defaultHeight: Int, val defau
     fun getMaxHeight(): Float = files.maxOfOrNull {
         OCFileUtils.getImageSize(it, defaultHeight.toFloat()).second.toFloat()
     } ?: 0f
-    fun calculateHashCode(): Long = files.sumOf { it.hashCode() }.toLong()
 }
diff --git a/app/src/main/java/com/owncloud/android/datamodel/ThumbnailsCacheManager.java b/app/src/main/java/com/owncloud/android/datamodel/ThumbnailsCacheManager.java
index 08250553fd6c..2213a77b7439 100644
--- a/app/src/main/java/com/owncloud/android/datamodel/ThumbnailsCacheManager.java
+++ b/app/src/main/java/com/owncloud/android/datamodel/ThumbnailsCacheManager.java
@@ -100,8 +100,8 @@ public final class ThumbnailsCacheManager {
     private static final String ETAG = "ETag";
 
     private static final Object mThumbnailsDiskCacheLock = new Object();
-    private static DiskLruImageCache mThumbnailCache;
-    private static boolean mThumbnailCacheStarting = true;
+    private static volatile DiskLruImageCache mThumbnailCache;
+    private static volatile boolean mThumbnailCacheStarting = true;
 
     private static final int DISK_CACHE_SIZE = 1024 * 1024 * 200; // 200MB
     private static final CompressFormat mCompressFormat = CompressFormat.JPEG;
@@ -1243,8 +1243,12 @@ public static void generateThumbnailFromOCFile(OCFile file, User user, Context c
 
     @VisibleForTesting
     public static void clearCache() {
-        mThumbnailCache.clearCache();
-        mThumbnailCache = null;
+        synchronized (mThumbnailsDiskCacheLock) {
+            if (mThumbnailCache != null) {
+                mThumbnailCache.clearCache();
+                mThumbnailCache = null;
+            }
+        }
     }
 
     public static void setClient(OwnCloudClient client) {
diff --git a/app/src/main/java/com/owncloud/android/ui/adapter/GalleryAdapter.kt b/app/src/main/java/com/owncloud/android/ui/adapter/GalleryAdapter.kt
index f7f0bc2ef76b..edbbe0d4a19d 100644
--- a/app/src/main/java/com/owncloud/android/ui/adapter/GalleryAdapter.kt
+++ b/app/src/main/java/com/owncloud/android/ui/adapter/GalleryAdapter.kt
@@ -106,7 +106,7 @@ class GalleryAdapter(
     }
 
     private fun updateFilesCount() {
-        cachedFilesCount = files.fold(0) { acc, item -> acc + item.rows.size }
+        cachedFilesCount = files.sumOf { it.rows.sumOf { it.files.size } }
     }
 
     private fun rebuildFilePositionMap() {
diff --git a/app/src/test/java/com/owncloud/android/ui/adapter/GalleryAdapterTest.kt b/app/src/test/java/com/owncloud/android/ui/adapter/GalleryAdapterTest.kt
index eca3acfef8ed..2b7bb193a56e 100644
--- a/app/src/test/java/com/owncloud/android/ui/adapter/GalleryAdapterTest.kt
+++ b/app/src/test/java/com/owncloud/android/ui/adapter/GalleryAdapterTest.kt
@@ -8,6 +8,7 @@
 package com.owncloud.android.ui.adapter
 
 import android.content.Context
+import android.text.TextUtils
 import com.nextcloud.client.account.User
 import com.nextcloud.client.jobs.upload.FileUploadHelper
 import com.nextcloud.client.preferences.AppPreferences
@@ -18,14 +19,18 @@ import com.owncloud.android.datamodel.OCFile
 import com.owncloud.android.ui.activity.ComponentsGetter
 import com.owncloud.android.ui.interfaces.OCFileListFragmentInterface
 import com.owncloud.android.utils.theme.ViewThemeUtils
-import junit.framework.Assert.assertEquals
+import io.mockk.every
+import io.mockk.mockkStatic
 import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
 import org.junit.Before
 import org.junit.Test
 import org.mockito.Mock
 import org.mockito.MockitoAnnotations
 import org.mockito.kotlin.doReturn
 import org.mockito.kotlin.whenever
+import kotlin.random.Random
 
 class GalleryAdapterTest {
     @Mock
@@ -55,20 +60,25 @@ class GalleryAdapterTest {
     private lateinit var mocks: AutoCloseable
 
     @Before
-    fun setUp() {
+    fun setUpMocks() {
         mocks = MockitoAnnotations.openMocks(this)
+
+        whenever(transferServiceGetter.storageManager) doReturn storageManager
+        whenever(transferServiceGetter.fileUploaderHelper) doReturn fileUploadHelper
+
+        // Mocking TextUtils so OCFile#existsOnDevice() doesn't fail due to Android not being available
+        // This is needed so OCFile#toString() works for logging errors
+        mockkStatic(TextUtils::class)
+        every { TextUtils.isEmpty(any()) } answers { arg<String?>(0)?.isEmpty() ?: true }
     }
 
     @After
-    fun tearDown() {
+    fun tearDownMocks() {
         mocks.close()
     }
 
     @Test
     fun testItemCount() {
-        whenever(transferServiceGetter.storageManager) doReturn storageManager
-        whenever(transferServiceGetter.fileUploaderHelper) doReturn fileUploadHelper
-
         val thumbnailSize = 50
 
         val sut = GalleryAdapter(
@@ -95,6 +105,113 @@ class GalleryAdapterTest {
 
         sut.addFiles(list)
 
-        assertEquals(2, sut.getFilesCount())
+        assertEquals(4, sut.getFilesCount())
+    }
+
+    @Test
+    @Suppress("LongMethod")
+    fun testIdUniqueness() {
+        val thumbnailSize = 50
+
+        val sut = GalleryAdapter(
+            context,
+            user,
+            ocFileListFragmentInterface,
+            preferences,
+            transferServiceGetter,
+            viewThemeUtils,
+            5,
+            thumbnailSize
+        )
+        val rows = mutableListOf<GalleryRow>()
+
+        // Test a known (former) hash collision
+        val row1File1 = 263512L
+        val row1File2 = 148830L
+        val row2File1 = 279897L
+        val row2File2 = 132445L
+        rows.add(
+            GalleryRow(
+                listOf(
+                    OCFile("/$row1File1.md").apply {
+                        fileId = row1File1
+                        parentId = 0
+                    },
+                    OCFile("/$row1File2.md").apply {
+                        fileId = row1File2
+                        parentId = 0
+                    }
+                ),
+                thumbnailSize,
+                thumbnailSize
+            )
+        )
+        rows.add(
+            GalleryRow(
+                listOf(
+                    OCFile("/$row2File1.md").apply {
+                        fileId = row2File1
+                        parentId = 0
+                    },
+                    OCFile("/$row2File2.md").apply {
+                        fileId = row2File2
+                        parentId = 0
+                    }
+                ),
+                thumbnailSize,
+                thumbnailSize
+            )
+        )
+        val alreadyUsedFileIds = listOf(row1File1, row1File2, row2File1, row2File2)
+
+        // Generate some random Ids for some explorative testing
+        val randomFileIds = uniquePositiveRandomLongs(10000, 1000000)
+        for (i in 0..<randomFileIds.size step 2) {
+            val id1 = randomFileIds[i]
+            val id2 = randomFileIds[i + 1]
+            if (id1 in alreadyUsedFileIds || id2 in alreadyUsedFileIds) {
+                continue
+            }
+            rows.add(
+                GalleryRow(
+                    listOf(
+                        OCFile("/$id1.md").apply { fileId = id1 },
+                        OCFile("/$id2.md").apply { fileId = id2 }
+                    ),
+                    thumbnailSize,
+                    thumbnailSize
+                )
+            )
+        }
+
+        val list = listOf(GalleryItems(1649317247, rows))
+        sut.addFiles(list)
+
+        val itemIds = mutableMapOf<Long, GalleryRow>()
+        for (i in 0..<sut.sectionCount) {
+            for (j in 0..<list[i].rows.size) {
+                val itemId = sut.getItemId(i, j)
+
+                assertFalse(
+                    "Item IDs are not unique: $itemId, in section $i, row $j is already taken by ${itemIds[itemId]}",
+                    itemIds.contains(itemId)
+                )
+                itemIds[itemId] = list[i].rows[j]
+            }
+        }
+        assertEquals("Exactly one section required, else the next assert is imprecise", 1, sut.sectionCount)
+        assertEquals("The duplicate detection seems to be faulty", rows.size, itemIds.size)
+    }
+
+    fun uniquePositiveRandomLongs(count: Int, max: Long = Long.MAX_VALUE): List<Long> {
+        require(count >= 0) { "count must be non-negative" }
+        if (count == 0) return emptyList()
+
+        val set = HashSet<Long>(count)
+        while (set.size < count) {
+            // produce positive (> 0) values
+            set.add(Random.nextLong(1, max))
+        }
+        return set.toList()
     }
 }