make user sheets popovers on desktop

lib/helpers/extensions/global_rect.dart

@@ -0,0 +1,15 @@
+import "package:material_ui/material_ui.dart";
+
+extension GlobalRect on BuildContext {
+  /// Where the widget this context belongs to is on screen, e.g. to open a
+  /// menu or popover next to it.
+  Rect get globalRect {
+    final overlay = Overlay.of(this).context.findRenderObject() as RenderBox;
+
+    return switch (findRenderObject()) {
+      final RenderBox box =>
+        box.localToGlobal(.zero, ancestor: overlay) & box.size,
+      _ => Rect.zero,
+    };
+  }
+}

lib/helpers/extensions/show_adaptive_menu.dart

@@ -1,4 +1,5 @@
 import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/global_rect.dart";
 import "package:nexus/helpers/extensions/is_desktop.dart";
 
 extension ShowAdaptiveMenu on BuildContext {
@@ -38,11 +39,7 @@ extension ShowAdaptiveMenu on BuildContext {
     final overlay = Overlay.of(this).context.findRenderObject() as RenderBox;
     final anchor = switch (globalPosition) {
       final position? => position & Size.zero,
-      null => switch (findRenderObject()) {
-        final RenderBox box =>
-          box.localToGlobal(.zero, ancestor: overlay) & box.size,
-        _ => Rect.zero,
-      },
+      null => globalRect,
     };
 
     await showMenu(

lib/helpers/extensions/show_adaptive_popover.dart

@@ -0,0 +1,35 @@
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/global_rect.dart";
+import "package:nexus/helpers/extensions/is_desktop.dart";
+import "package:nexus/helpers/extensions/show_adaptive_sheet.dart";
+import "package:nexus/widgets/popover_layout.dart";
+
+extension ShowAdaptivePopover on BuildContext {
+  /// Shows the widget from [builder] in a popover next to the widget this
+  /// context belongs to on desktop, or a bottom sheet on mobile. Use the
+  /// context passed to [builder] to close it.
+  Future<void> showAdaptivePopover(WidgetBuilder builder) {
+    if (!isDesktop) return showAdaptiveSheet(builder);
+
+    final anchor = globalRect;
+
+    return showDialog(
+      context: this,
+      barrierColor: Colors.transparent,
+      useSafeArea: false,
+      builder: (context) => CustomSingleChildLayout(
+        delegate: PopoverLayout(anchor),
+        child: ConstrainedBox(
+          constraints: const .new(maxWidth: 420, maxHeight: 600),
+          child: Material(
+            elevation: 8,
+            clipBehavior: .antiAlias,
+            color: Theme.of(context).colorScheme.surfaceContainer,
+            borderRadius: .circular(16),
+            child: Builder(builder: builder),
+          ),
+        ),
+      ),
+    );
+  }
+}

lib/helpers/extensions/show_user_popover.dart

@@ -1,5 +1,5 @@
 import "package:material_ui/material_ui.dart";
-import "package:nexus/helpers/extensions/show_adaptive_sheet.dart";
+import "package:nexus/helpers/extensions/show_adaptive_popover.dart";
 import "package:nexus/models/content/membership.dart";
 import "package:nexus/widgets/user_bottom_sheet.dart";
 
@@ -8,7 +8,7 @@ extension ShowUserPopover on BuildContext {
     MembershipContent member,
     String userId, {
     String? roomId,
-  }) => showAdaptiveSheet(
+  }) => showAdaptivePopover(
     (context) => UserBottomSheet(member, userId, roomId: roomId),
   );
 }

lib/widgets/member_list.dart

@@ -106,8 +106,16 @@ class const MemberList(final String roomId, {super.key})
                                       MembershipContent(
                                             :final avatarUrl,
                                             :final displayName,
-                                      ) =>
-                                        ListTile(
+                                          ) &&
+                                          final content =>
+                                        Builder(
+                                          builder: (context) => ListTile(
+                                            onTap: () =>
+                                                context.showUserPopover(
+                                                  content,
+                                                  members[index].stateKey!,
+                                                  roomId: roomId,
+                                                ),
                                             title: Text(
                                               displayName ??
                                                   members[index]
@@ -137,21 +145,11 @@ class const MemberList(final String roomId, {super.key})
                                               ),
                                             ),
                                           ),
+                                        ),
                                       _ => throw Exception(
                                         "Member content was not MembershipContent",
                                       ),
                                     },
-                                onTap: (index) {
-                                  final member = members[index];
-                                  if (member.content
-                                      case MembershipContent content) {
-                                    context.showUserPopover(
-                                      content,
-                                      member.stateKey!,
-                                      roomId: roomId,
-                                    );
-                                  }
-                                },
                               ),
                             ];
                           }).toList(),

lib/widgets/popover_layout.dart

@@ -0,0 +1,33 @@
+import "dart:math";
+
+import "package:material_ui/material_ui.dart";
+
+/// Positions a popover below [anchor], or above it if there isn't room below,
+/// keeping it fully on screen.
+class const PopoverLayout(final Rect anchor) extends SingleChildLayoutDelegate {
+  static const margin = 8.0;
+
+  @override
+  BoxConstraints getConstraintsForChild(BoxConstraints constraints) =>
+      constraints.loosen().deflate(const .all(margin));
+
+  @override
+  Offset getPositionForChild(Size size, Size childSize) {
+    final fitsBelow = anchor.bottom + childSize.height + margin <= size.height;
+
+    return .new(
+      anchor.left.clamp(
+        margin,
+        max(margin, size.width - childSize.width - margin),
+      ),
+      (fitsBelow ? anchor.bottom : anchor.top - childSize.height).clamp(
+        margin,
+        max(margin, size.height - childSize.height - margin),
+      ),
+    );
+  }
+
+  @override
+  bool shouldRelayout(PopoverLayout oldDelegate) =>
+      oldDelegate.anchor != anchor;
+}