Add animation on the timeline content, to avoid glitch (#1323)

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/animation/AlphaAnimation.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.element.android.libraries.designsystem.animation
+
+import androidx.compose.animation.core.animateFloatAsState
+import androidx.compose.animation.core.tween
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.State
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.platform.LocalInspectionMode
+
+@Composable
+fun alphaAnimation(
+    fromAlpha: Float = 0f,
+    toAlpha: Float = 1f,
+    delayMillis: Int = 150,
+    durationMillis: Int = 150,
+    label: String = "AlphaAnimation",
+): State<Float> {
+    val firstAlpha = if (LocalInspectionMode.current) 1f else fromAlpha
+    var alpha by remember { mutableFloatStateOf(firstAlpha) }
+    LaunchedEffect(Unit) { alpha = toAlpha }
+    return animateFloatAsState(
+        targetValue = alpha,
+        animationSpec = tween(
+            delayMillis = delayMillis,
+            durationMillis = durationMillis,
+        ),
+        label = label
+    )
+}