fix(ui): make the module tab strip a scrollable single column

src/LageBuch.App.Shared/Theme/Styles.axaml

@@ -283,6 +283,33 @@
   <Style Selector="TabControl">
     <Setter Property="TabStripPlacement" Value="Left" />
     <Setter Property="Padding" Value="0" />
+    <!-- Avalonia's default Left-placement items panel is a vertical WrapPanel with no scroll
+         wrapper at all: once the rail runs out of height it wraps into a second column overlapping
+         the first two tabs with the last two (reported live on a short window — WASSERFÖRDERUNG
+         rendered beside AUFBAU, ABBAU beside ETB), and forcing a single-column panel alone just
+         turns that into a silent, unreachable clip instead (confirmed via a headless test: no
+         ScrollViewer anywhere wraps PART_ItemsPresenter, so a short window drops the last tabs off
+         the bottom edge with no way back). This is a single-column "dispatch rail" by design (fixed
+         172px-wide TabItems below) with only one placement ever used in this app, so replace the
+         whole template with a purpose-built one: single-column items panel, wrapped in its own
+         ScrollViewer, laid out beside the content host. -->
+    <Setter Property="ItemsPanel">
+      <ItemsPanelTemplate>
+        <StackPanel Orientation="Vertical" />
+      </ItemsPanelTemplate>
+    </Setter>
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Grid ColumnDefinitions="Auto,*">
+          <ScrollViewer Grid.Column="0" HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto">
+            <ItemsPresenter Name="PART_ItemsPresenter" ItemsPanel="{TemplateBinding ItemsPanel}" />
+          </ScrollViewer>
+          <ContentPresenter Grid.Column="1" Name="PART_SelectedContentHost"
+                            Content="{TemplateBinding SelectedContent}"
+                            ContentTemplate="{TemplateBinding SelectedContentTemplate}" />
+        </Grid>
+      </ControlTemplate>
+    </Setter>
   </Style>
   <Style Selector="TabControl /template/ ItemsPresenter#PART_ItemsPresenter">
     <Setter Property="Margin" Value="0" />

tests/LageBuch.Acceptance.Tests/ModuleTabStripLayoutTests.cs

@@ -0,0 +1,81 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+using LageBuch.App.Shared.Views;
+using LageBuch.AppLogic;
+using LageBuch.AppLogic.Services;
+using LageBuch.AppLogic.ViewModels;
+
+namespace LageBuch.Acceptance.Tests;
+
+// The left "dispatch sidebar" module tab strip (AUFBAU, ETB, ..., WASSERFÖRDERUNG, ABBAU),
+// TabStripPlacement="Left" per Theme/Styles.axaml. On a short window, the last tabs wrapped into
+// a second column overlapping the first ones instead of scrolling -- reported live, screenshot
+// showed WASSERFÖRDERUNG rendered directly beside AUFBAU and ABBAU beside ETB.
+public class ModuleTabStripLayoutTests
+{
+    // CanHost=true so the header includes the "IM NETZWERK FREIGEBEN" row, matching the reported
+    // screenshot exactly (a NoopIncidentHostController hides that row, understating header height).
+    private sealed class HostableController : IIncidentHostController
+    {
+        public bool CanHost => true;
+        public bool IsHosting => false;
+        public string? ShareHint => null;
+        public string? SharePin => null;
+        public Task StartAsync(LocalIncidentSession session) => Task.CompletedTask;
+        public Task StopAsync() => Task.CompletedTask;
+    }
+
+    [AvaloniaFact]
+    public void Tabs_stay_in_a_single_column_top_to_bottom_on_a_short_window()
+    {
+        var clock = new FixedClock();
+        var session = LocalIncidentSession.StartNew(new FakeStore(), clock,
+            new LageBuch.Domain.SessionOperator("Müller", "FFB 12/1"), "/x.fwincident",
+            Array.Empty<(string, bool)>(), Array.Empty<(string, bool)>());
+        var vm = new IncidentWorkspaceViewModel(session, clock, new ManualTicker(),
+            WorkspaceRenderHelper.MasterData(), new FakeDialogs(), new NoopAlarmService(),
+            new HostableController());
+
+        // Matches the reported "relative small screen": a modest laptop-class height, well under
+        // what 11 fixed-MinHeight-50 TabItems need stacked below the full header.
+        var window = new Window { Content = new IncidentWorkspaceView { DataContext = vm }, Width = 1366, Height = 550 };
+        window.Show();
+        Dispatcher.UIThread.RunJobs();
+
+        var tabItems = window.GetVisualDescendants().OfType<TabItem>()
+            .Select(t => t.TranslatePoint(new Point(0, 0), window)!.Value.Y)
+            .ToList();
+
+        Assert.True(tabItems.Count >= 10, $"Expected all module tabs to be found, got {tabItems.Count}.");
+        for (var i = 1; i < tabItems.Count; i++)
+        {
+            Assert.True(tabItems[i] > tabItems[i - 1],
+                $"Tab at index {i} (y={tabItems[i]}) is not below the previous tab (y={tabItems[i - 1]}) -- " +
+                "the tab strip wrapped into a second column instead of staying single-column.");
+        }
+
+        // Not wrapping alone isn't enough -- the last tabs must still be reachable by scrolling
+        // the rail into view, not merely pushed off-screen with no way back.
+        var lastTab = window.GetVisualDescendants().OfType<TabItem>().Last(); // ABBAU
+        var railScrollViewer = window.GetVisualDescendants().OfType<ScrollViewer>()
+            .FirstOrDefault(sv => sv.GetVisualDescendants().OfType<TabItem>().Any());
+        Assert.NotNull(railScrollViewer);
+        Assert.True(railScrollViewer!.Extent.Height > railScrollViewer.Viewport.Height,
+            "Test premise: the rail must actually need scrolling at this window height " +
+            $"(extent={railScrollViewer.Extent.Height}, viewport={railScrollViewer.Viewport.Height}).");
+
+        railScrollViewer.Offset = new Vector(0, railScrollViewer.Extent.Height - railScrollViewer.Viewport.Height);
+        Dispatcher.UIThread.RunJobs();
+        var lastTabY = lastTab.TranslatePoint(new Point(0, 0), window)!.Value.Y;
+        Assert.InRange(lastTabY, 0, window.Bounds.Height);
+
+        var dir = Path.Combine(Path.GetTempPath(), "lagebuch-shots");
+        Directory.CreateDirectory(dir);
+        using var frame = window.CaptureRenderedFrame()!;
+        frame.SavePng(Path.Combine(dir, "module-tab-strip-small-screen-fixed.png"));
+    }
+}