Flutter: How to render a popup widget on top globally without loosing interactions with other widgets?

There’s a widget for that.

See the screen recording of the code example below

Try Autocomplete widget which works well for the search ui you explained.

A widget for helping the user make a selection by entering some text and choosing from among a list of options. more

For the popup ui, See Overlay widget.

Overlays let independent child widgets "float" visual elements on top of other widgets by inserting them into the overlay’s stack. The overlay lets each of these widgets manage their participation in the overlay using OverlayEntry objects.

The Overlay widget uses a custom stack implementation, which is very similar to the Stack widget. The main use case of Overlay is related to navigation and being able to insert widgets on top of the pages in an app. For layout purposes unrelated to navigation, consider using Stack instead.

In the example below. The OverlayEntry is the widget that is being drawn (The widget that you wanted to popup). It inserted in to the OverlaySate. To anchor the overlay (popup) widget to another, it’s wrapped with CompositedTransformFollower and the target widget with CompositedTransformTarget, with LayerLink class linking them. This helps the popup follow along when scrolling.

Note: The overlay method literally overlays the whole app screen. So, it may draw over sticky widgets like the appbar or bottom nav.

import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';

void main() => runApp(const App());

class App extends StatelessWidget {
  const App({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Material(
        child: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              titleAutoComplete,
              spaceSmall,
              const AutoCompleteExample(),
              spaceMedium,
              textLorem20,
              spaceMedium,
              titleCustomAutoComplete,
              spaceSmall,
              const CustomAutoCompleteExample(),
              spaceMedium,
              textLorem20,
              spaceMedium,
              titleOverlay,
              spaceSmall,
              const OverlayExample(),
            ],
          ),
        ),
      ),
    );
  }
}

class OverlayExample extends StatefulWidget {
  const OverlayExample({super.key});

  @override
  State<OverlayExample> createState() => _OverlayExampleState();
}

class _OverlayExampleState extends State<OverlayExample> {
  // A place in an [Overlay] that can contain a widget.
  late OverlayEntry overlayEntry;
  // The current state of an [Overlay].
  late OverlayState overlayState;
  // The link object that connects this [CompositedTransformTarget]
  // with one or more [CompositedTransformFollower]s.
  final layerLink = LayerLink();

  @override
  void initState() {
    super.initState();
    overlayEntry = buildOverlay();
    overlayState = Overlay.of(context);
  }

  @override
  void dispose() {
    // Make sure to remove OverlayEntry when the widget is disposed.
    overlayEntry.remove();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return CompositedTransformTarget(
      link: layerLink,
      child: ElevatedButton(
        onPressed: () => overlayState.insert(overlayEntry),
        child: const Text('Click Me'),
      ),
    );
  }

  buildOverlay() => OverlayEntry(builder: (_) {
        return Positioned(
          width: 100,
          height: 100,
          child: CompositedTransformFollower(
            link: layerLink,
            targetAnchor: Alignment.topCenter,
            followerAnchor: Alignment.bottomLeft,
            child: const FlutterLogo(),
          ),
        );
      });
}

class AutoCompleteExample extends StatelessWidget {
  const AutoCompleteExample({super.key});

  static const List<String> colors = <String>[
    'red',
    'green',
    'blue',
    'purple',
    'yellow',
    'orange'
  ];

  @override
  Widget build(BuildContext context) {
    return Autocomplete<String>(
      fieldViewBuilder: fieldViewBuilder,
      optionsBuilder: (TextEditingValue textEditingValue) {
        if (textEditingValue.text == '') {
          return const Iterable<String>.empty();
        }
        return colors.where((String option) {
          return option.contains(textEditingValue.text.toLowerCase());
        });
      },
      onSelected: (String selection) {
        debugPrint('You just selected $selection');
      },
    );
  }
}

class CustomAutoCompleteExample extends StatelessWidget {
  const CustomAutoCompleteExample({super.key});

  @override
  Widget build(BuildContext context) {
    return Autocomplete<MapEntry<String, Color>>(
      fieldViewBuilder: fieldViewBuilder,
      optionsBuilder: (TextEditingValue textEditingValue) {
        if (textEditingValue.text == '') {
          return const Iterable<MapEntry<String, Color>>.empty();
        }
        return colorsMap.entries.where((MapEntry<String, Color> element) {
          return element.key.contains(textEditingValue.text.toLowerCase());
        });
      },
      displayStringForOption: (option) {
        return option.key;
      },
      onSelected: (MapEntry<String, Color> selection) {
        debugPrint('You just selected $selection');
      },
      optionsViewBuilder: (context, onSelected, options) {
        return Align(
          alignment: Alignment.topLeft,
          child: Material(
            elevation: 1,
            child: ListView.builder(
              shrinkWrap: true,
              itemCount: options.length,
              itemBuilder: (BuildContext context, int index) {
                final option = options.elementAt(index);
                return InkWell(
                  onTap: () => onSelected(option),
                  child: Builder(
                    builder: (BuildContext context) {
                      final bool highlight =
                          AutocompleteHighlightedOption.of(context) == index;
                      if (highlight) {
                        SchedulerBinding.instance
                            .addPostFrameCallback((Duration timeStamp) {
                          Scrollable.ensureVisible(context, alignment: 0.5);
                        });
                      }
                      return Container(
                        color: highlight ? Theme.of(context).focusColor : null,
                        padding: const EdgeInsets.all(16.0),
                        child: Row(
                          children: [
                            CircleAvatar(backgroundColor: option.value),
                            const SizedBox(width: 24),
                            Text(option.key),
                          ],
                        ),
                      );
                    },
                  ),
                );
              },
            ),
          ),
        );
      },
    );
  }
}

// Constants and stylings

const spaceSmall = SizedBox(height: 8.0);
const spaceMedium = SizedBox(height: 16.0);

var titleAutoComplete = const Text(
  'AutoComplete Example',
  style: textStyle,
);

var titleCustomAutoComplete = const Text(
  'Custom AutoComplete Example',
  style: textStyle,
);
var titleOverlay = const Text(
  'Overlay Example',
  style: textStyle,
);

const textLorem20 = Text(
    'Lorem ipsum dolor sit amet consectetur adipisicing elit. Quas fugiat tempore commodi quo quam repellat ipsum blanditiis sapiente at nobis!');

const textStyle = TextStyle(fontSize: 20, fontWeight: FontWeight.bold);

const Map<String, Color> colorsMap = {
  'red': Colors.red,
  'green': Colors.green,
  'blue': Colors.blue,
  'purple': Colors.purple,
  'yellow': Colors.yellow,
  'orange': Colors.orange,
};

Widget fieldViewBuilder(
  context,
  textEditingController,
  focusNode,
  onFieldSubmitted,
) {
  return TextFormField(
    controller: textEditingController,
    focusNode: focusNode,
    onEditingComplete: onFieldSubmitted,
    decoration: InputDecoration(
      hintText: 'Search Color',
      border: OutlineInputBorder(
        borderRadius: BorderRadius.circular(
          8,
        ),
      ),
    ),
  );
}