make user sheets popovers on desktop

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;
+}