Open a group by double-clicking it, and say where you are

ONE PRESS WAS DOING TWO JOBS. A group card was the only place a group could be
selected — it is what EDIT and DELETE aim at — and it was also the control that
narrowed the grid to that group. So there was no way to name a group in order to
rename it without every host outside it leaving the screen at the same moment,
and no way back except a SHOW ALL button that appeared beside the heading.

Two gestures instead. A click selects and does nothing else; a double-click
opens, which is what the host cards below already do to get a shell and what the
transfers screen's directories already do to go inside one. The grid now has one
vocabulary rather than one per list. The gesture is wired in the code-behind
beside the host one, and guarded the same way: a double-click on the space
around the cards must not open whichever group happened to be selected.

THE CARDS ARE ONE LEVEL NOW, not every group in the keychain. Groups nest, and
drawing all of them flat was the only honest thing to do while a card was a
filter — a filter nobody can see is a filter nobody can turn off. Once opening
one became navigation the cards became its contents, and VisibleGroups is that
level beside Groups the way VisibleHosts sits beside Hosts. Groups itself is
untouched: it is what every lookup reads and what the phone's headings are built
from, and the phone binds none of the new members.

Which is what the trail is for. A level with no name and no way out is a grid
that has quietly hidden things, so a breadcrumb sits above the cards — drawn
exactly as the transfers screen draws a directory path, same flat crumbs and
same separator, because it is the same control answering the same question and a
window with two breadcrumbs that look different has two ideas of what a path is.
The first crumb is always there and always goes back to every host, which is
what SHOW ALL was; that button went with it, because a control that only says
"stop" beside a trail that says where you are is a second control for one job,
and this one also gets you back one level rather than all the way.

EDIT AND DELETE AIM AT GroupTarget: the selected card, or the open group when no
card is selected. Without the fallback a group with nothing inside it could be
opened and then never edited, because opening a group is exactly what takes its
own card off the screen. It is also what a file manager does — act on the
selection, and on the current folder when there is none — and the pair is now
hidden with nothing to act on rather than shown doing nothing.

A DANGLING PARENT AND A CYCLE BOTH HAD TO END UP SOMEWHERE REACHABLE. Neither is
prevented: a parent id may point at a group deleted on another machine, and two
clients can each re-parent A under B and B under A while offline, which no merge
can see because the pointer is inside the payload. EffectiveParents promotes
both to the outermost level, which is the same degradation the resolver's
visited set produces for inheritance. The repair for either is the group's own
editor and the editor is opened from its card, so a group drawn nowhere would be
a broken state with the fix locked inside it.

Three tests in HostGridTests: the split rule through the properties the cards
bind, the pointer gesture itself in two windows so that "one press still only
selects" is asserted separately from the pair, and a nested group opened,
emptied of cards and walked back out of one level. The last presses the trail as
it is actually rendered rather than calling the command, because a crumb reaches
the vault through a $parent binding — a string that compiles whether or not it
resolves, and would otherwise leave a row of buttons that do nothing. 85 layout
tests and 234 shell-flow tests pass.

Manual-checks 3.2, 7.6 and 7.7 follow the new gestures, and 3.2a and 3.4a are
new: nesting, and the two states above, both of which need two machines and
neither of which headless Avalonia can reach.

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

@@ -177,15 +177,13 @@
 
               <!--
                 The group's own two actions, beside the group cards rather than in the toolbar, because they
-                act on the selected card and this is where the selection is made. Hidden rather than
-                disabled while DELETE's question is up, as every other pair in this application is, so it
-                cannot be pressed twice — see VaultViewModel.ShowsGroupActions.
+                act on the card that is selected — and this is where the selection is made. Hidden rather
+                than disabled while DELETE's question is up, as every other pair in this application is, so
+                it cannot be pressed twice, and hidden again when there is nothing for them to act on. What
+                that is, with no card selected, is the group the trail below ends with; see
+                VaultViewModel.GroupTarget.
               -->
               <StackPanel Grid.Column="2" Orientation="Horizontal" Spacing="6">
-                <Button Classes="ghost" Content="SHOW ALL"
-                        Command="{Binding ClearGroupFilterCommand}"
-                        IsVisible="{Binding IsFilteredByGroup}"
-                        ToolTip.Tip="Stops narrowing the grid to one group." />
                 <Button Classes="ghost" Content="EDIT" Command="{Binding EditGroupCommand}"
                         IsVisible="{Binding ShowsGroupActions}" />
                 <Button Classes="ghost" Content="DELETE" Command="{Binding DeleteGroupCommand}"
@@ -193,6 +191,43 @@
               </StackPanel>
             </Grid>
 
+            <!--
+              ============ WHERE YOU ARE ============
+              The trail, drawn exactly as the transfers screen draws a directory path, because it is the same
+              control answering the same question — and a window with two breadcrumbs that look different has
+              two ideas of what a path is. See TransfersScreen.axaml.
+
+              It exists because the cards below are one level of a tree rather than every group at once, and
+              a level nobody can name is a grid that has quietly hidden things. The first crumb is always
+              there and always goes back to every host, which is what the SHOW ALL button used to be: a
+              button that only says "stop" beside a trail that says where you are is the second control for
+              the same job, and this one also gets you halfway back rather than all the way.
+            -->
+            <ItemsControl ItemsSource="{Binding GroupTrail}">
+              <ItemsControl.ItemsPanel>
+                <ItemsPanelTemplate><WrapPanel Orientation="Horizontal" /></ItemsPanelTemplate>
+              </ItemsControl.ItemsPanel>
+              <ItemsControl.ItemTemplate>
+                <DataTemplate x:DataType="vm:GroupCrumbViewModel">
+                  <StackPanel Orientation="Horizontal">
+                    <!--
+                      The command is the vault's and the item is a crumb, so it is reached through the
+                      ItemsControl's data context — the same way every other templated command on the
+                      desktop is bound.
+                    -->
+                    <Button Classes="flat" Padding="3,1"
+                            Command="{Binding $parent[ItemsControl].((vm:VaultViewModel)DataContext).OpenGroupCommand}"
+                            CommandParameter="{Binding Group}">
+                      <TextBlock Classes="mono" Text="{Binding Name}" FontSize="11"
+                                 Foreground="{StaticResource TextDim}" />
+                    </Button>
+                    <TextBlock Classes="mono" Text="›" FontSize="11"
+                               Foreground="{StaticResource TextFaint}" VerticalAlignment="Center" />
+                  </StackPanel>
+                </DataTemplate>
+              </ItemsControl.ItemTemplate>
+            </ItemsControl>
+
             <Border Padding="10" Background="{StaticResource DangerWash}" CornerRadius="6"
                     IsVisible="{Binding IsConfirmingGroupDeletion}">
               <views:ConfirmDeleteCard />
@@ -204,21 +239,28 @@
               every other list in this application draws a selected row. Buttons would have needed a
               Classes.active binding per card and a second copy of "which one".
 
-              SelectedItem is GroupFilter and not SelectedGroup. The two are nearly the same thing here —
-              GroupFilter assigns SelectedGroup — but only one of them may narrow the grid; see the property
-              for why the phone's own use of SelectedGroup must not.
+              ◆ ONE CLICK SELECTS, TWO OPEN. SelectedItem is the selection and nothing else now — the
+              gesture that opens a group is a double-click, wired in the code-behind beside the one that
+              opens a shell on a host. One click used to mean both, and a card was then the only place a
+              group could be named while also being the control that threw the rest of the grid away.
+
+              ◆ THIS IS ONE LEVEL, NOT EVERY GROUP. It binds VisibleGroups: what is inside the group the
+              trail above ends with, or the outermost groups when it ends at ALL HOSTS. Folded away entirely
+              at a group with nothing inside it, which is an ordinary thing to open — the trail and the two
+              buttons stay, because leaving it is a gesture and editing it is a button.
 
               ◆ THESE CARDS ARE THE DROP TARGET. A host card dragged onto one is filed under that group, and
               that is the whole of what a drag does on this screen. It used to be a heading inside the host
               grid, and the headings are gone — see the note on the host grid below. A group is a thing that
-              is drawn once, at the top, where the eye already goes to narrow the grid; a target that is also
-              the label of what you are aiming at needs no explaining. See the code-behind for the gesture.
+              is drawn once, at the top, where the eye already goes; a target that is also the label of what
+              you are aiming at needs no explaining. See the code-behind for the gesture.
             -->
             <ListBox Classes="tiles" x:Name="GroupGrid"
                      DragDrop.AllowDrop="True"
-                     ItemsSource="{Binding Groups}"
-                     SelectedItem="{Binding GroupFilter}"
-                     ToolTip.Tip="Press a group to show only its hosts. Drag a host card onto one to file it there.">
+                     IsVisible="{Binding HasVisibleGroups}"
+                     ItemsSource="{Binding VisibleGroups}"
+                     SelectedItem="{Binding SelectedGroup}"
+                     ToolTip.Tip="Press a group to select it, double-press to open it. Drag a host card onto one to file it there.">
               <ListBox.ItemsPanel>
                 <ItemsPanelTemplate><WrapPanel /></ItemsPanelTemplate>
               </ListBox.ItemsPanel>