Let the recovery code be copied, and give the phone a clipboard to copy to

Both screens had made the code selectable and both said why: a person who cannot
get it out of the box photographs the screen, and a screenshot is a worse home
for it than a clipboard. This finishes that argument. Selecting 64 characters of
letter-spaced monospace with a thumb is the version of "possible" people give up
on halfway — and on the phone the screen blocks screenshots, so the honest
remaining options were retyping it or losing it.

It is the one secret this application deliberately offers to a clipboard, and the
contrast with the keychain's copy is the whole argument rather than an
inconsistency. There, copying the private half is refused outright, because
installing a key means pasting the public one and the private one has no business
leaving the vault. Here there is no better route: the code exists for one screen,
is stored nowhere, and has to reach a password manager. The clipboard is the
intended destination rather than a way round the design.

The sentence afterwards matters as much as the copy, and is asserted: a clipboard
is a staging post, this screen is the only place the code exists, and the next
thing copied replaces it. Somebody who copies and does nothing has not saved it.

The phone had no clipboard delegate at all — the desktop passed one and this head
passed null — so COPY PUBLIC KEY on the keychain answered "this machine has no
clipboard" on a device that plainly has one. Nothing about that was platform
shaped: Android has a clipboard and Avalonia surfaces it through the same
TopLevel. Wiring it fixes that copy too.

The test fixture built its shell without a clipboard, which modelled the bug
rather than the product, so it has one now and the public-key test asserts what
lands there instead of the refusal. The refusal keeps its own test, on a shell
built without one, because the view model reads the delegate's absence rather
than an empty result — and because a button that silently does nothing on this
screen is worse than one that refuses.

src/DodoSSH.Client.Android/App.axaml.cs

@@ -1,5 +1,7 @@
 using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Input.Platform;
 using Avalonia.Markup.Xaml;
 
 using DodoSSH.Client.Android.Platform;
@@ -79,7 +81,14 @@ public sealed partial class DodoSshApp : Avalonia.Application
         // RESUME has something to read — and whatever a process death interrupted. See DocumentStaging.
         DocumentStaging.Sweep();
 
-        var viewModel = ComposeShell(paths, caches, workspace, knownHosts, connections);
+        // Built before the view model, because the clipboard is reached through it — see ClipboardWriter,
+        // which takes the surface and looks the TopLevel up on each call rather than now.
+        var shell = new PhoneShell();
+
+        var viewModel = ComposeShell(
+            paths, caches, workspace, knownHosts, connections, ClipboardWriter(shell));
+
+        shell.DataContext = viewModel;
 
         // Difference 2: the foreground service, which is what makes TerminalWorkspace's promise — that a
         // shell outlives a vault lock — true on a platform that stops backgrounded processes.
@@ -104,9 +113,40 @@ public sealed partial class DodoSshApp : Avalonia.Application
 
         keepAlive.Refresh();
 
-        return new PhoneShell { DataContext = viewModel };
+        return shell;
     }
 
+    /// <summary>
+    /// Writing to this phone's clipboard.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// ◆ <b>This head had none, and every control that wanted one said so out loud.</b> COPY PUBLIC KEY on
+    /// the keychain answered "This machine has no clipboard" on a device that plainly has one, because the
+    /// delegate was simply never wired here — the desktop passed one and the phone passed null. Android has
+    /// a clipboard and Avalonia surfaces it through the same <c>TopLevel</c> the desktop reaches, so there
+    /// was nothing platform-shaped about the gap.
+    /// </para>
+    /// <para>
+    /// Looked up per call rather than captured, exactly as the desktop's is: at composition there is no
+    /// <c>TopLevel</c> yet, because this is the method building the view it will be attached to. A machine
+    /// that somehow has none falls through silently and the view model decides what to say, which is what
+    /// keeps "no clipboard here" and "copied" different answers.
+    /// </para>
+    /// <para>
+    /// A delegate rather than an <c>IClipboard</c>, so nothing in the view models needs a visual and every
+    /// test that drives them stays window-free.
+    /// </para>
+    /// </remarks>
+    private static Func<string, Task> ClipboardWriter(Visual surface) =>
+        async text =>
+        {
+            if (TopLevel.GetTopLevel(surface) is { Clipboard: { } clipboard })
+            {
+                await clipboard.SetTextAsync(text).ConfigureAwait(false);
+            }
+        };
+
     /// <remarks>
     /// Split from <see cref="Compose"/> only for length. The division is a real one though: above this is
     /// the platform graph, and below it is the shell every head shares.
@@ -116,7 +156,8 @@ public sealed partial class DodoSshApp : Avalonia.Application
         ClientCacheFactory caches,
         TerminalWorkspace workspace,
         VaultKnownHostStore knownHosts,
-        SshNetConnectionFactory connections)
+        SshNetConnectionFactory connections,
+        Func<string, Task> copyToClipboard)
     {
         // Difference 3: the Android keystore, with a fingerprint or the device credential releasing the
         // key. A straight implementation of the interface the session layer has always taken.
@@ -165,6 +206,8 @@ public sealed partial class DodoSshApp : Avalonia.Application
             // would name the same machine. See PhoneEnvironment.DeviceName.
             deviceName: PhoneEnvironment.DeviceName,
 
+            copyToClipboard: copyToClipboard,
+
             updates: updates);
 
         // Started rather than awaited: framework initialisation must not block on a schema migration. The

src/DodoSSH.Client.Android/Views/RecoveryCodeScreen.axaml

@@ -44,6 +44,26 @@
                              LetterSpacing="1.5" LineHeight="26" TextWrapping="Wrap" />
       </Border>
 
+      <!--
+        ◆ COPY, and on this head it is the difference between selectable and reachable. The comment above
+        says a user who cannot get the code out of the box photographs the screen; selecting a monospaced,
+        letter-spaced string with a thumb is the version of "can" that people give up on halfway, and this
+        screen blocks screenshots — so without a button the honest options were retyping it or losing it.
+
+        Full width and above the tick rather than beside the code, because it is a step in the sequence
+        this screen walks somebody through: read it, take it, confirm you have it. A small icon in the
+        corner of the box would be the same control drawn as an afterthought.
+
+        What it puts on the clipboard is the one secret this application deliberately offers there — see
+        MainWindowViewModel.CopyRecoveryCode, which sets out why that is right here and wrong for a private
+        key, and what the status line has to say afterwards.
+      -->
+      <Button Classes="secondary" Content="COPY CODE" Margin="0,10,0,0" Height="44"
+              Command="{Binding CopyRecoveryCodeCommand}" />
+
+      <TextBlock Classes="detail" Margin="0,8,0,0" TextWrapping="Wrap"
+                 Text="{Binding StatusMessage}" />
+
       <CheckBox IsChecked="{Binding RecoveryCodeWrittenDown}" Margin="0,20,0,0" MinHeight="44">
         <TextBlock Classes="body" Foreground="{StaticResource Text}"
                    Text="I have written this code down. I understand it will never be shown again." />

src/DodoSSH.Client.App/Views/MainWindow.axaml

@@ -297,8 +297,24 @@
                                      FontSize="17" Foreground="{StaticResource Accent}"
                                      TextWrapping="Wrap" />
               </Border>
-              <CheckBox IsChecked="{Binding RecoveryCodeWrittenDown}"
-                        Content="I have written it down somewhere safe" />
+              <!--
+                ◆ COPY, beside the tick rather than inside the box. The code is selectable and has been
+                since this card existed, on the argument that refusing to let somebody take it only pushes
+                them to a screenshot — this is that argument finished, because "select 64 characters of
+                letter-spaced monospace without missing one" is a thing people get wrong silently.
+
+                It is the one secret this application offers to the clipboard on purpose; see
+                MainWindowViewModel.CopyRecoveryCode for why that is right here and refused for a private
+                key, and for what the status line below says afterwards.
+              -->
+              <StackPanel Orientation="Horizontal" Spacing="10">
+                <Button Classes="ghost" Content="COPY CODE"
+                        Command="{Binding CopyRecoveryCodeCommand}"
+                        ToolTip.Tip="Puts the code on the clipboard. Paste it into your password manager now — it is shown once and the next thing you copy replaces it." />
+                <CheckBox IsChecked="{Binding RecoveryCodeWrittenDown}"
+                          Content="I have written it down somewhere safe"
+                          VerticalAlignment="Center" />
+              </StackPanel>
               <Button Classes="accent" Content="CONTINUE" Command="{Binding ConfirmRecoveryCodeCommand}"
                       HorizontalAlignment="Left" />
               <TextBlock Classes="hint" Text="{Binding StatusMessage}" />

src/DodoSSH.Client.Shell/ViewModels/MainWindowViewModel.cs

@@ -1848,6 +1848,52 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
             }).ConfigureAwait(true);
     }
 
+    /// <summary>
+    /// Puts the recovery code on the clipboard.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// ◆ <b>The one secret in this application that is deliberately offered to the clipboard, and the
+    /// contrast with <c>VaultViewModel.CopyPublicKeyAsync</c> is the whole argument.</b> There, copying the
+    /// <em>private</em> key is refused outright, because installing a key means pasting the public half and
+    /// the private one has no business leaving the vault. Here there is no better route: the code exists for
+    /// one screen, is stored nowhere, and has to reach a password manager — so the clipboard is the intended
+    /// destination rather than a way around the design.
+    /// </para>
+    /// <para>
+    /// Both screens already made the code selectable, and both said why: a person who cannot get it out of
+    /// the box photographs the screen, and a screenshot is a far worse home for it than a clipboard. This is
+    /// that argument finished. Selecting a monospaced, letter-spaced string with a thumb is the version of
+    /// "possible" that people give up on.
+    /// </para>
+    /// <para>
+    /// It says what it did, including the case where there is nothing to say it to — a machine with no
+    /// clipboard has to be told so rather than left with a button that appears to do nothing, which is the
+    /// same rule the keychain's copy already follows. And the sentence names what has to happen next,
+    /// because a clipboard is not somewhere a recovery code may stay: this screen is the only moment it
+    /// exists, and the next thing copied replaces it.
+    /// </para>
+    /// </remarks>
+    [RelayCommand]
+    private async Task CopyRecoveryCodeAsync()
+    {
+        if (RecoveryCode is not { Length: > 0 } code)
+        {
+            return;
+        }
+
+        if (copyToClipboard is null)
+        {
+            StatusMessage = "This machine has no clipboard. Select the code and copy it by hand.";
+            return;
+        }
+
+        await copyToClipboard(code).ConfigureAwait(true);
+
+        StatusMessage = "Copied. Paste it into your password manager now — this screen is the only place "
+            + "it exists, and the next thing you copy replaces it.";
+    }
+
     /// <summary>Leaves the recovery-code screen, once the user says they have it.</summary>
     [RelayCommand]
     private void ConfirmRecoveryCode()