Fix marking a room as read re-instantiates its timeline (#5628)

* Add `Timeline.markAsRead` to avoid reinstantiating the timeline using `Room.markAsRead`

* Mark as read when exiting the room screen, destroy the timeline when fully closed

* Ensure `MarkAsFullyReadAndExit` event can only be processed once

* Fix `DelayedVisibility` not being displayed in previews

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/utils/DelayedVisibility.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2025 New Vector 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.designsystem.utils
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.movableContentOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.platform.LocalInspectionMode
+import kotlinx.coroutines.delay
+import kotlin.time.Duration
+import kotlin.time.Duration.Companion.milliseconds
+
+/**
+ * Displays the content of [block] after a delay of [duration].
+ */
+@Composable
+fun DelayedVisibility(
+    duration: Duration = 300.milliseconds,
+    block: @Composable () -> Unit,
+) {
+    // Technically this shouldn't be needed because `LocalInspectionMode` won't change, but let's make the linter happy
+    val movableBlock = remember { movableContentOf { block() } }
+    if (LocalInspectionMode.current) {
+        // Just allow the contents to be displayed in the previews/screenshot tests
+        movableBlock()
+    } else {
+        var shouldDisplay by remember { mutableStateOf(false) }
+        LaunchedEffect(Unit) {
+            delay(duration)
+            shouldDisplay = true
+        }
+        AnimatedVisibility(shouldDisplay) {
+            movableBlock()
+        }
+    }
+}