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),
   );
 }