Selected users: if scrollable, make last one peek

If there are enough selected users that they can't all be displayed,
add extra padding in between the users to ensure that the last
visible one is half visible to provide some scroll affordance.

libraries/matrixui/src/main/kotlin/io/element/android/libraries/matrix/ui/components/SelectedUsersList.kt

@@ -16,18 +16,26 @@
 
 package io.element.android.libraries.matrix.ui.components
 
+import androidx.compose.foundation.border
 import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.LazyRow
-import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.lazy.rememberLazyListState
 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.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.layout.Layout
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
 import io.element.android.libraries.designsystem.preview.ElementPreviewDark
@@ -35,6 +43,7 @@ import io.element.android.libraries.designsystem.preview.ElementPreviewLight
 import io.element.android.libraries.matrix.api.user.MatrixUser
 import kotlinx.collections.immutable.ImmutableList
 import kotlinx.collections.immutable.toImmutableList
+import kotlin.math.floor
 
 @Composable
 fun SelectedUsersList(
@@ -56,16 +65,60 @@ fun SelectedUsersList(
         }
     }
 
+    val rowWidth by remember {
+        derivedStateOf {
+            lazyListState.layoutInfo.viewportSize.width - lazyListState.layoutInfo.beforeContentPadding
+        }
+    }
+
     LazyRow(
         state = lazyListState,
-        modifier = modifier,
+        modifier = modifier
+            .fillMaxWidth(),
         contentPadding = contentPadding,
-        horizontalArrangement = Arrangement.spacedBy(24.dp),
     ) {
-        items(selectedUsers.toList()) { matrixUser ->
-            SelectedUser(
-                matrixUser = matrixUser,
-                onUserRemoved = onUserRemoved,
+        itemsIndexed(selectedUsers.toList()) { index, matrixUser ->
+            Layout(
+                content = {
+                    SelectedUser(
+                        matrixUser = matrixUser,
+                        onUserRemoved = onUserRemoved,
+                    )
+                },
+                measurePolicy = { measurables, constraints ->
+                    // Measures out extra space between each user to ensure that the last visible user is exactly half visible, giving an affordance that
+                    // the user can scroll to see more.
+
+                    val placeable = measurables.first().measure(constraints)
+                    val isLastItem = index == selectedUsers.lastIndex
+                    val width = if (isLastItem) {
+                        // Don't add any padding on the final item
+                        placeable.width
+                    } else {
+                        val minimumSpacing = 24.dp.toPx()
+                        val userWidth = placeable.width + minimumSpacing
+                        val maxVisibleUsers = rowWidth / userWidth
+                        if (maxVisibleUsers >= selectedUsers.size) {
+                            // If we can fit all the users in, don't do anything fancy
+                            (placeable.width + minimumSpacing).toInt()
+                        } else {
+                            // Round down the number of visible users to end with a state where one is half visible
+                            val targetFraction = (placeable.width / 2) / userWidth
+                            val targetUsers = floor(maxVisibleUsers - targetFraction) + targetFraction
+
+                            // Work out how much extra spacing we need to reduce the number of users that much, then split it evenly amongst the visible users
+                            val extraSpacing = (maxVisibleUsers - targetUsers) * userWidth
+                            val extraSpacingPerUser = (extraSpacing) / floor(targetUsers)
+
+                            (placeable.width + minimumSpacing + extraSpacingPerUser).toInt()
+                        }
+                    }
+
+
+                    layout(width, placeable.height) {
+                        placeable.place(0, 0)
+                    }
+                }
             )
         }
     }
@@ -81,7 +134,23 @@ internal fun SelectedUsersListDarkPreview() = ElementPreviewDark { ContentToPrev
 
 @Composable
 private fun ContentToPreview() {
-    SelectedUsersList(
-        selectedUsers = aMatrixUserList().take(6).toImmutableList(),
-    )
+    Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        // Two users that will be visible with no scrolling
+        SelectedUsersList(
+            selectedUsers = aMatrixUserList().take(2).toImmutableList(),
+            modifier = Modifier
+                .width(200.dp)
+                .border(1.dp, Color.Red)
+        )
+
+        // Multiple users that don't fit, so will be spaced out per the measure policy
+        for (i in 0..5) {
+            SelectedUsersList(
+                selectedUsers = aMatrixUserList().take(6).toImmutableList(),
+                modifier = Modifier
+                    .width((200 + (i * 20)).dp)
+                    .border(1.dp, Color.Red)
+            )
+        }
+    }
 }