fix: correct re-centering animation making flicker

shared/src/commonMain/kotlin/fr/ajaury/gwenedeg/player/ui/SubtitleList.kt

@@ -18,8 +18,9 @@ import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.snapshotFlow
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
@@ -27,6 +28,7 @@ import androidx.compose.ui.text.font.FontStyle
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
 import fr.ajaury.gwenedeg.core.model.Phrase
 import fr.ajaury.gwenedeg.preferences.model.SubtitlePreferences
 import fr.ajaury.gwenedeg.subtitle.model.SubtitleLine
@@ -44,19 +46,24 @@ fun SubtitleList(
 ) {
     val listState = rememberLazyListState()
     val isDragged by listState.interactionSource.collectIsDraggedAsState()
+    var targetIndex by remember { mutableStateOf(0) }
+    var isDragging by remember { mutableStateOf(false) }
 
-    val centeredIndex by remember {
+    // Detect beginning of dragging.
+    LaunchedEffect(isDragged) {
+        if (isDragged) {
+            isDragging = true
+        }
+    }
+
+    val centeredIndex by remember(currentIndex, isDragged) {
         derivedStateOf {
-            val info = listState.layoutInfo
-            if (info.visibleItemsInfo.isEmpty()) {
-                -1
+            if (isDragging) {
+                listState.getCenteredIndex().also {
+                    targetIndex = it
+                }
             } else {
-                val viewportCenter = (info.viewportStartOffset + info.viewportEndOffset) / 2f
-                info.visibleItemsInfo
-                    .minByOrNull {
-                        abs((it.offset + it.size / 2f) - viewportCenter)
-                    }?.index
-                    ?: -1
+                currentIndex ?: -1
             }
         }
     }
@@ -69,21 +76,15 @@ fun SubtitleList(
     }
 
     // Snap to the centered line and seek to it once the user finishes scrolling.
-    LaunchedEffect(listState, lines) {
-        var wasDragged = false
-        snapshotFlow { isDragged to listState.isScrollInProgress }
-            .collect { (dragged, scrolling) ->
-                if (dragged) {
-                    wasDragged = true
-                } else if (wasDragged && !scrolling) {
-                    wasDragged = false
-                    val targetIndex = centeredIndex
-                    if (targetIndex in lines.indices) {
-                        listState.centerItem(targetIndex)
-                        onSeek(targetIndex)
-                    }
-                }
+    LaunchedEffect(isDragged) {
+        if (isDragging && !isDragged) {
+            if (targetIndex in lines.indices) {
+                listState.centerItem(targetIndex)
+                onSeek(targetIndex)
             }
+
+            isDragging = false
+        }
     }
 
     BoxWithConstraints(modifier = modifier) {
@@ -107,6 +108,20 @@ fun SubtitleList(
     }
 }
 
+private fun LazyListState.getCenteredIndex(): Int {
+    val info = this.layoutInfo
+    return if (info.visibleItemsInfo.isEmpty()) {
+        -1
+    } else {
+        val viewportCenter = (info.viewportStartOffset + info.viewportEndOffset) / 2f
+        info.visibleItemsInfo
+            .minByOrNull {
+                abs((it.offset + it.size / 2f) - viewportCenter)
+            }?.index
+            ?: -1
+    }
+}
+
 private suspend fun LazyListState.centerItem(index: Int) {
     if (index < 0 || index >= this.layoutInfo.totalItemsCount) return
 
@@ -126,11 +141,8 @@ private fun Subtitle(
     isCurrent: Boolean,
     subtitlePreferences: SubtitlePreferences = SubtitlePreferences(),
 ) {
-    val contentColor = if (isCurrent) {
-        Color.Unspecified
-    } else {
-        MaterialTheme.colorScheme.onSurface.copy(alpha = 0.5f)
-    }
+    val textAlpha = if (isCurrent) 1f else 0.35f
+    val contentColor = MaterialTheme.colorScheme.onSurface.copy(alpha = textAlpha)
 
     Column(
         modifier = Modifier.fillMaxWidth(),
@@ -140,7 +152,6 @@ private fun Subtitle(
         if (subtitlePreferences.showTranscription) {
             TranscriptionLine(
                 text = line.phrase.transcription,
-                isCurrent = isCurrent,
                 contentColor = contentColor,
             )
         }
@@ -149,7 +160,6 @@ private fun Subtitle(
             ?.let { translation ->
                 TranslationLine(
                     text = translation,
-                    isCurrent = isCurrent,
                     contentColor = contentColor,
                 )
             }
@@ -159,18 +169,11 @@ private fun Subtitle(
 @Composable
 private fun TranscriptionLine(
     text: String,
-    isCurrent: Boolean,
     contentColor: Color,
 ) {
-    val mainTextStyle = if (isCurrent) {
-        MaterialTheme.typography.headlineSmall
-    } else {
-        MaterialTheme.typography.bodyLarge
-    }
-
     Text(
         text = text,
-        style = mainTextStyle,
+        style = MaterialTheme.typography.bodyLarge.copy(fontSize = 20.sp),
         color = contentColor,
         textAlign = TextAlign.Center,
         modifier = Modifier.fillMaxWidth(),
@@ -180,18 +183,11 @@ private fun TranscriptionLine(
 @Composable
 private fun TranslationLine(
     text: String,
-    isCurrent: Boolean,
     contentColor: Color,
 ) {
-    val translationStyle = if (isCurrent) {
-        MaterialTheme.typography.bodyLarge
-    } else {
-        MaterialTheme.typography.bodySmall
-    }
-
     Text(
         text = text,
-        style = translationStyle.copy(fontStyle = FontStyle.Italic),
+        style = MaterialTheme.typography.bodySmall.copy(fontStyle = FontStyle.Italic),
         color = contentColor,
         textAlign = TextAlign.Center,
         modifier = Modifier.fillMaxWidth(),