Fix copy buttons on the web build over plain HTTP

lib/clipboard_interop/clipboard_interop.dart

@@ -0,0 +1,16 @@
+import 'clipboard_io.dart' if (dart.library.js_interop) 'clipboard_web.dart';
+
+/// Puts [text] on the system clipboard.
+///
+/// Wraps `Clipboard.setData` everywhere except the web, where it throws over
+/// plain HTTP: the engine's only strategy is `navigator.clipboard`, which
+/// browsers expose in a secure context alone, and `http://admin.awl` is not
+/// one. The web branch keeps that path and falls back to `execCommand`, which
+/// carries no such requirement.
+///
+/// Throws if the copy did not happen, which callers must handle: a copy button
+/// that reports success it didn't have sends the user off to paste an empty
+/// clipboard.
+Future<void> copyText(String text) async {
+  return copyTextImpl(text);
+}

lib/clipboard_interop/clipboard_io.dart

@@ -0,0 +1,8 @@
+import 'package:flutter/services.dart';
+
+/// The default branch, and a real implementation rather than a stub: this is a
+/// framework call with no `dart:io` in it, so it serves Android, iOS and
+/// desktop alike.
+Future<void> copyTextImpl(String text) async {
+  await Clipboard.setData(ClipboardData(text: text));
+}

lib/clipboard_interop/clipboard_web.dart

@@ -0,0 +1,49 @@
+import 'dart:js_interop';
+
+import 'package:web/web.dart' as web;
+
+/// `navigator.clipboard` is absent outside a secure context, but package:web
+/// types it non-nullable because the IDL says it is always there. Reading it
+/// through a nullable view is what lets the two cases be told apart.
+extension on web.Navigator {
+  @JS('clipboard')
+  external web.Clipboard? get clipboardOrNull;
+}
+
+Future<void> copyTextImpl(String text) async {
+  // First, because it is the one that works in a secure context (localhost in
+  // development, any HTTPS deployment) and execCommand is deprecated.
+  final clipboard = web.window.navigator.clipboardOrNull;
+  if (clipboard != null) {
+    await clipboard.writeText(text).toDart;
+    return;
+  }
+
+  // Ported from the Flutter engine's own ExecCommandCopyStrategy (BSD-3),
+  // which served this case from 2020 until https://github.com/flutter/flutter/pull/171427 removed it
+  // as redundant with the Clipboard API, counting browser versions and not
+  // insecure contexts. execCommand needs a user gesture, which a button is.
+  final textArea = web.HTMLTextAreaElement();
+  // Off-screen and transparent rather than hidden: `display: none` cannot be
+  // selected, and selection is what execCommand copies.
+  textArea.style
+    ..position = 'absolute'
+    ..top = '-99999px'
+    ..left = '-99999px'
+    ..opacity = '0'
+    ..color = 'transparent'
+    ..backgroundColor = 'transparent'
+    ..background = 'transparent';
+  web.document.body!.appendChild(textArea);
+  textArea.value = text;
+  // Without preventScroll the page jumps to the invisible textarea.
+  textArea.focus(web.FocusOptions(preventScroll: true));
+  textArea.select();
+  try {
+    if (!web.document.execCommand('copy')) {
+      throw StateError('Clipboard copy was refused by the browser');
+    }
+  } finally {
+    textArea.remove();
+  }
+}

lib/common.dart

@@ -1,7 +1,8 @@
 import 'dart:async';
 
+import 'package:anywherelan/clipboard_interop/clipboard_interop.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:qr_flutter/qr_flutter.dart';
 import 'package:share_plus/share_plus.dart';
 
@@ -91,16 +92,19 @@ class ExitNodePermissionField extends StatelessWidget {
   }
 }
 
+const copyFailedMessage = kIsWeb
+    ? 'Could not copy. Select the text and press Ctrl+C.'
+    : 'Could not copy to clipboard.';
+
 /// A value the user is meant to take away rather than read: a link, a peer ID.
 /// Rendered as a field so that seventy-odd characters of base58 read as a value
 /// with a copy button on it instead of as a paragraph.
 ///
 /// The value stays **in full and selectable**, however many lines that takes,
 /// and copying is a button beside it rather than a tap on the field. Truncating
-/// to one line is tidier and must not happen: on the web build served over
-/// plain HTTP `Clipboard.setData` throws (`navigator.clipboard` is
-/// secure-context-only), so hand-selecting the text is the only copy path that
-/// works there, and it needs the whole value on screen.
+/// to one line is tidier and must not happen: a browser can refuse a
+/// programmatic copy, and hand-selecting the text is then the only path left,
+/// which needs the whole value on screen.
 ///
 /// The button answers in place, the tick replacing the icon for two seconds: a
 /// [SnackBar] raised inside a dialog lands at the bottom of the screen, far
@@ -161,7 +165,13 @@ class _CopyableFieldState extends State<CopyableField> {
   }
 
   Future<void> _copy() async {
-    await Clipboard.setData(ClipboardData(text: widget.value));
+    try {
+      await copyText(widget.value);
+    } catch (_) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(const SnackBar(content: Text(copyFailedMessage)));
+      return;
+    }
     if (!mounted) return;
 
     ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(widget.copiedMessage)));

lib/peer_settings_screen.dart

@@ -1,10 +1,10 @@
 import 'dart:async';
 
+import 'package:anywherelan/clipboard_interop/clipboard_interop.dart';
 import 'package:anywherelan/common.dart';
 import 'package:anywherelan/entities.dart';
 import 'package:anywherelan/providers.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 /// Adapter for [PeerSettingsView] that loads the peer config from the
@@ -332,8 +332,17 @@ class _PeerSettingsViewState extends State<PeerSettingsView> {
                 suffixIcon: IconButton(
                   icon: Icon(Icons.content_copy),
                   tooltip: 'Copy Peer ID',
-                  onPressed: () {
-                    Clipboard.setData(ClipboardData(text: widget.peerConfig.peerId));
+                  onPressed: () async {
+                    try {
+                      await copyText(widget.peerConfig.peerId);
+                    } catch (_) {
+                      if (!mounted) return;
+                      ScaffoldMessenger.of(
+                        context,
+                      ).showSnackBar(const SnackBar(content: Text(copyFailedMessage)));
+                      return;
+                    }
+                    if (!mounted) return;
                     ScaffoldMessenger.of(
                       context,
                     ).showSnackBar(SnackBar(content: Text("Peer ID copied to clipboard")));

lib/status_tab.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 
+import 'package:anywherelan/clipboard_interop/clipboard_interop.dart';
 import 'package:anywherelan/common.dart';
 import 'package:anywherelan/connection_error.dart';
 import 'package:anywherelan/entities.dart';
@@ -8,7 +9,6 @@ import 'package:anywherelan/qr_dialog.dart';
 import 'package:anywherelan/server_interop/server_interop.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 /// Adapter for [StatusPageView] that reads [myPeerInfoProvider] and
@@ -754,7 +754,13 @@ class _AddressField extends StatelessWidget {
           icon: const Icon(Icons.copy_rounded),
           tooltip: 'Copy address',
           onPressed: () async {
-            await Clipboard.setData(ClipboardData(text: address));
+            try {
+              await copyText(address);
+            } catch (_) {
+              if (!context.mounted) return;
+              ScaffoldMessenger.of(context).showSnackBar(const SnackBar(content: Text(copyFailedMessage)));
+              return;
+            }
             if (!context.mounted) return;
             ScaffoldMessenger.of(
               context,

test/widget/copyable_field_test.dart

@@ -48,6 +48,29 @@ void main() {
     expect(shared, 1);
   });
 
+  testWidgets('says the copy failed instead of confirming one that did not happen', (tester) async {
+    // What the web build over plain HTTP does when the browser refuses: the
+    // button used to leave no trace at all, the throw going nowhere.
+    tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, (call) async {
+      if (call.method == 'Clipboard.setData') {
+        throw PlatformException(code: 'copy_fail', message: 'Clipboard is not available in the context.');
+      }
+      return null;
+    });
+    addTearDown(
+      () => tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, null),
+    );
+
+    await pumpField(tester);
+    await tester.tap(find.byIcon(Icons.content_copy));
+    await tester.pump();
+
+    expect(find.text(copyFailedMessage), findsOneWidget);
+    expect(find.text('Link copied to clipboard'), findsNothing);
+    // No tick either: it is the same claim of success in another form.
+    expect(find.byIcon(Icons.check), findsNothing);
+  });
+
   testWidgets('copies the value and says so, share or no share', (tester) async {
     String? copied;
     tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, (call) async {

test/widget/peer_settings_screen_test.dart

@@ -1,6 +1,8 @@
+import 'package:anywherelan/common.dart';
 import 'package:anywherelan/entities.dart';
 import 'package:anywherelan/peer_settings_screen.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../fixtures/fixture_reader.dart';
@@ -41,6 +43,28 @@ void main() {
       expect(tester.widget<Switch>(switchFinder).value, cfg.weAllowUsingAsExitNode);
     });
 
+    testWidgets('does not report a Peer ID copy that failed', (tester) async {
+      // This button used to announce success before the copy was even awaited,
+      // so on a build where the clipboard is unreachable it sent the user off
+      // to paste an empty clipboard.
+      tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, (call) async {
+        if (call.method == 'Clipboard.setData') {
+          throw PlatformException(code: 'copy_fail');
+        }
+        return null;
+      });
+      addTearDown(
+        () => tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, null),
+      );
+
+      await pumpAppWidget(tester, PeerSettingsView(peerConfig: _loadConfig()), size: desktopSize);
+      await tester.tap(find.widgetWithIcon(IconButton, Icons.content_copy));
+      await tester.pump();
+
+      expect(find.text('Peer ID copied to clipboard'), findsNothing);
+      expect(find.text(copyFailedMessage), findsOneWidget);
+    });
+
     testWidgets('no invite marker for a peer added the usual way', (tester) async {
       await pumpAppWidget(tester, PeerSettingsView(peerConfig: _loadConfig()), size: desktopSize);