Open a new host in the vault of the group it is being made in

+ NEW HOST decided two defaults separately and let them contradict each other.
The group came from the screen — the selected card, or failing that the group
whose contents are showing — and the vault came from the keychain screen's
standing "new items go to" preference. Inside a group belonging to any other
vault the two disagreed, and the group is what lost: GroupInEditingVault drops a
group the editor's vault has not got, on the sound reasoning that a host filed
under an id its readers cannot resolve looks unfiled to everybody but the person
who wrote it. So pressing the button while standing inside a team's PLATFORM
opened a form filed under nothing, bound for the personal vault, with no sentence
anywhere saying either thing had happened.

The vault now follows the group. A group lives in exactly one vault, so a host
that is to land in that group has to be sealed in that vault too — which is the
rule + NEW GROUP has followed for a parent since the cards became a tree, and the
comment there claiming this as a deliberate difference from the host's editor is
the one the code has now caught up with.

The filter stays, because there is one case left for it: the group's vault may be
one this session can read and not write, a team vault this account is a viewer of.
TargetVaults is the readable-and-writable set and is what decides here, so a
viewer keeps the standing preference and loses the group with it, rather than
opening an editor aimed at a save that cannot happen.

Both directions are tested, since one alone would not say which default wins:
standing in a shared vault's group, the editor opens on that vault with the group
selected and the host saves there; and with the preference pointed at the shared
vault while a personal-vault group is open, the group beats the picker somebody
set once.

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

@@ -6174,10 +6174,10 @@ internal sealed partial class VaultViewModel(
 
         editingEntityId = null;
 
-        // The keychain screen's picker is the default rather than the answer: the editor has a picker of its
-        // own from here on, and moving that one is what decides where this host lands. See
-        // EditorVaultChoices.
-        editingHostVaultId = TargetVaultId;
+        // Read before the vault is decided, because it is what decides it.
+        var inherited = GroupTarget;
+
+        editingHostVaultId = VaultForANewHostUnder(inherited);
 
         EditorLabel = string.Empty;
         EditorHostname = string.Empty;
@@ -6199,11 +6199,11 @@ internal sealed partial class VaultViewModel(
         // that the group whose contents are showing. Adding three machines to the group somebody has just
         // made is the ordinary case, and since the grid holds one level at a time the alternative is worse
         // than a default nobody chose: a host created inside a group and filed under none would vanish from
-        // the screen it was created on. Only when that group is in the vault this host is going into,
-        // though — the grid draws the active vault's groups, and inheriting one into a shared vault would
-        // file the host under something nobody else in it can resolve. Before the authentication picker,
-        // because whether there is a group to inherit from decides whether that one offers to.
-        BuildGroupChoices(GroupInEditingVault(GroupTarget?.EntityId));
+        // the screen it was created on. Still filtered by the vault being written to, which is that group's
+        // own wherever this session can write there: what the filter is left holding is the case where it
+        // cannot. Before the authentication picker, because whether there is a group to inherit from decides
+        // whether that one offers to.
+        BuildGroupChoices(GroupInEditingVault(inherited?.EntityId));
 
         BuildAuthenticationChoices(
             boundKeyId: null,
@@ -6214,6 +6214,32 @@ internal sealed partial class VaultViewModel(
         Status = "Adding a host.";
     }
 
+    /// <summary>Which vault a host started from the grid is sealed in.</summary>
+    /// <param name="inherited">The group the editor is about to open on, if there is one.</param>
+    /// <remarks>
+    /// <para>
+    /// The keychain screen's picker is the default rather than the answer: the editor has a picker of its own
+    /// from here on, and moving that one is what decides where the host lands. See
+    /// <see cref="EditorVaultChoices"/>.
+    /// </para>
+    /// <para>
+    /// A group to inherit beats it, because a group lives in exactly one vault and a host that is to land in
+    /// that group has to be sealed in that vault too. Deciding the two separately made them contradict each
+    /// other: the group was dropped by <see cref="GroupInEditingVault"/> and + NEW HOST inside PLATFORM
+    /// opened a form filed under nothing, bound for somewhere else. It is the rule <see cref="NewGroup"/>
+    /// already follows for a parent.
+    /// </para>
+    /// <para>
+    /// Only where this session can write to that vault, which is what <see cref="TargetVaults"/> is asked
+    /// here: a viewer of a shared vault keeps the standing preference and loses the group with it, rather
+    /// than opening an editor aimed at a save that cannot happen.
+    /// </para>
+    /// </remarks>
+    private Guid VaultForANewHostUnder(HostGroupRowViewModel? inherited) =>
+        inherited is { } group && TargetVaults.Any(choice => choice.VaultId == group.VaultId)
+            ? group.VaultId
+            : TargetVaultId;
+
     /// <summary>Opens the selected host for editing.</summary>
     [RelayCommand]
     private void EditSelectedHost()
@@ -6623,11 +6649,10 @@ internal sealed partial class VaultViewModel(
     /// while the open group is the screen everybody can see they are on.
     /// </para>
     /// <para>
-    /// <b>And in the open group's vault, which is where this differs from <see cref="NewHost"/> a second
-    /// time.</b> A host opens on the standing "new items go to" preference and takes the open group only if
-    /// that group happens to be in the same vault; a group made inside another group is in that group's
-    /// vault by construction, because a parent in a second vault is a level half the readers cannot resolve.
-    /// Defaulting to the preference instead would answer "+ NEW GROUP inside PLATFORM" with a group
+    /// <b>And in the open group's vault</b>, which <see cref="NewHost"/> now does too and for the same
+    /// reason: a parent — or a group — in a second vault is a level half the readers cannot resolve, so the
+    /// thing being made goes where the thing it is going inside already is. Defaulting to the standing
+    /// "new items go to" preference instead would answer "+ NEW GROUP inside PLATFORM" with a group
     /// somewhere else and no parent — a form that silently dropped the one thing the button said.
     /// </para>
     /// </remarks>

tests/DodoSSH.Client.App.Tests/VaultSharingTests.cs

@@ -1619,6 +1619,80 @@ public sealed class VaultSharingTests : IAsyncLifetime
             .ShouldBeNull("the host is left where it was rather than filed under an unresolvable group");
     }
 
+    /// <remarks>
+    /// <para>
+    /// The same rule as the drag above, arrived at from the other end: a group lives in exactly one vault, so
+    /// + NEW HOST inside one has to open on that vault as well as on that group. The two defaults were
+    /// decided separately — the group came from the screen, the vault from the standing "new items go to"
+    /// preference — so pressing the button inside a shared vault's group opened a form bound for the personal
+    /// vault, with the group silently dropped by the picker that keeps the two in step.
+    /// </para>
+    /// <para>
+    /// The second half is the same test read backwards, and it is what says which of the two wins. The
+    /// preference is an answer for a host being made from nowhere in particular; the group somebody is
+    /// standing inside is a better one, and it is the one the button was pressed in.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ANewHostInsideAGroup_IsMadeInThatGroupsVaultRatherThanTheStandingPreference()
+    {
+        await UnlockedAsync();
+
+        var vaults = shell.Vaults;
+
+        await CreateVaultAsync(vaults, "Platform secrets");
+
+        var vault = shell.Vault!;
+        var sharedVaultId = vaults.SelectedVault!.VaultId;
+
+        await vault.LoadAsync(Token);
+
+        // In the personal vault, which is where a new group goes with nothing open and the preference
+        // untouched. It is the one the second half of this test stands in.
+        await AddGroupAsync(vault, "staging", under: null);
+
+        var personalVaultId = Named(vault, "staging").VaultId;
+
+        personalVaultId.ShouldNotBe(sharedVaultId, "this test is meaningless with one vault");
+
+        vault.NewGroupCommand.Execute(null);
+
+        vault.GroupEditorSelectedVault =
+            vault.GroupEditorVaultChoices.Single(choice => choice.VaultId == sharedVaultId);
+
+        vault.GroupEditorLabel = "production";
+        await vault.SaveGroupCommand.ExecuteAsync(null);
+
+        vault.OpenGroupCommand.Execute(Named(vault, "production"));
+
+        vault.NewHostCommand.Execute(null);
+
+        vault.EditorSelectedVault.ShouldNotBeNull(vault.Status).VaultId
+            .ShouldBe(sharedVaultId, "the group the screen is standing in is in the shared vault");
+        vault.EditorSelectedGroup.ShouldNotBeNull().Label
+            .ShouldBe("production", "and the group survives, having a vault it can be resolved in");
+
+        vault.EditorLabel = "prod-db";
+        vault.EditorHostname = "db.internal";
+        await vault.SaveHostCommand.ExecuteAsync(null);
+
+        var host = vault.Hosts.ShouldHaveSingleItem();
+
+        host.VaultId.ShouldBe(sharedVaultId, vault.Status);
+        host.Host.GroupId.ShouldBe(Named(vault, "production").EntityId);
+
+        // And backwards: the preference names the shared vault and the open group is in the personal one.
+        vault.SelectedTargetVault = vault.TargetVaults.Single(choice => choice.VaultId == sharedVaultId);
+
+        vault.OpenGroupCommand.Execute(Named(vault, "staging"));
+
+        vault.NewHostCommand.Execute(null);
+
+        vault.EditorSelectedVault.ShouldNotBeNull(vault.Status).VaultId
+            .ShouldBe(personalVaultId, "the group somebody is standing in beats the picker they set once");
+        vault.EditorSelectedGroup.ShouldNotBeNull().Label.ShouldBe("staging");
+    }
+
     /// <remarks>
     /// The mirror image of the host test above, and it goes the other way on purpose. A host filed into a
     /// shared vault has to stay there, because hosts are read across every readable vault and so come back;