show bottom sheets / dialogs / menus based on screen size

Fixes #76

lib/helpers/extensions/show_adaptive_menu.dart

@@ -0,0 +1,55 @@
+import "package:material_ui/material_ui.dart";
+import "package:nexus/helpers/extensions/is_desktop.dart";
+
+extension ShowAdaptiveMenu on BuildContext {
+  /// Shows [items] in a popup menu on desktop, or a bottom sheet on mobile.
+  ///
+  /// On desktop, the menu opens at [globalPosition] if given (e.g. where the
+  /// user right clicked), or over the widget this context belongs to.
+  Future<void> showAdaptiveMenu(
+    List<PopupMenuEntry> items, {
+    Offset? globalPosition,
+  }) async {
+    if (!isDesktop) {
+      await showModalBottomSheet(
+        context: this,
+        isScrollControlled: true,
+        showDragHandle: true,
+        builder: (context) => SafeArea(
+          child: SingleChildScrollView(
+            padding: const .only(bottom: 8),
+            // PopupMenuItems must be inside something with the menu role
+            child: Semantics(
+              role: .menu,
+              explicitChildNodes: true,
+              child: ListTileTheme.merge(
+                contentPadding: const .symmetric(horizontal: 24),
+                minTileHeight: 48,
+                titleTextStyle: Theme.of(context).textTheme.titleMedium,
+                child: Column(mainAxisSize: .min, children: items),
+              ),
+            ),
+          ),
+        ),
+      );
+      return;
+    }
+
+    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,
+      },
+    };
+
+    await showMenu(
+      context: this,
+      constraints: .loose(.infinite),
+      position: .fromRect(anchor, Offset.zero & overlay.size),
+      items: items,
+    );
+  }
+}