fix(workspace): scroll sidebar instead of wrapping into columns on short viewports

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

@@ -283,6 +283,45 @@
   <Style Selector="TabControl">
     <Setter Property="TabStripPlacement" Value="Left" />
     <Setter Property="Padding" Value="0" />
+    <!-- The stock Fluent TabControl template seats its ItemsPresenter directly in a DockPanel.
+         With TabStripPlacement="Left" the theme v-flips the presenter's default WrapPanel, and on
+         a viewport too short to fit every rail tab the WrapPanel wraps into extra side-by-side
+         columns instead of overflowing. Wrapping the presenter in a ScrollViewer (which takes the
+         Dock role) gives the WrapPanel unbounded height in its wrap direction, so the rail stays a
+         single column and grows a scrollbar instead (#146). This is the upstream 12.1.1 template
+         with exactly that one change; PART_ItemsPresenter keeps its name so the theme's own
+         ^[TabStripPlacement=Left] /template/ ItemsPresenter#PART_ItemsPresenter > WrapPanel rule
+         and the app's Margin=0 rule below still find and apply to it. -->
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Border BorderBrush="{TemplateBinding BorderBrush}"
+                BorderThickness="{TemplateBinding BorderThickness}"
+                CornerRadius="{TemplateBinding CornerRadius}"
+                Background="{TemplateBinding Background}"
+                HorizontalAlignment="{TemplateBinding HorizontalAlignment}"
+                VerticalAlignment="{TemplateBinding VerticalAlignment}">
+          <DockPanel>
+            <ScrollViewer DockPanel.Dock="{TemplateBinding TabStripPlacement}"
+                          HorizontalScrollBarVisibility="Disabled"
+                          VerticalScrollBarVisibility="Auto">
+              <ItemsPresenter Name="PART_ItemsPresenter"
+                              ItemsPanel="{TemplateBinding ItemsPanel}" />
+            </ScrollViewer>
+            <Panel ClipToBounds="True">
+              <ContentPresenter Name="PART_SelectedContentHost2"
+                                Margin="{TemplateBinding Padding}"
+                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
+                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
+                                IsVisible="False" />
+              <ContentPresenter Name="PART_SelectedContentHost"
+                                Margin="{TemplateBinding Padding}"
+                                HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
+                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
+            </Panel>
+          </DockPanel>
+        </Border>
+      </ControlTemplate>
+    </Setter>
   </Style>
   <Style Selector="TabControl /template/ ItemsPresenter#PART_ItemsPresenter">
     <Setter Property="Margin" Value="0" />

tests/LageBuch.Acceptance.Tests/ModuleTabsScrollingTests.cs

@@ -0,0 +1,69 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Presenters;
+using Avalonia.Headless;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+using LageBuch.App.Shared.Views;
+
+namespace LageBuch.Acceptance.Tests;
+
+// The left nav rail is a TabControl with TabStripPlacement="Left". On a viewport too short to fit
+// all rail tabs vertically, Avalonia's WrapPanel (the default ItemsPanel, v-flipped by the theme for
+// Left placement) used to wrap into a second side-by-side column -- confusing on a phone-height
+// window, and the second column can run off the right edge entirely. These tests pin that the rail
+// stays in ONE column and overflows into a scrollbar instead of wrapping (#146).
+public class ModuleTabsScrollingTests
+{
+    // Short enough to force wrap regardless of which header banners are visible: 10 tabs at
+    // MinHeight=50 need 500px, and the header/footer stacks eat most of the rest.
+    private const double ShortHeight = 400.0;
+
+    [AvaloniaFact]
+    public void Nav_rail_stays_one_column_and_scrolls_on_a_short_viewport()
+    {
+        var vm = WorkspaceRenderHelper.BuildEditableWorkspaceWithAllBars();
+        var window = new Window
+        {
+            Content = new IncidentWorkspaceView { DataContext = vm },
+            Width = 1920,
+            Height = ShortHeight,
+        };
+        window.Show();
+        Dispatcher.UIThread.RunJobs();
+
+        // Capture BEFORE the assertions: on the buggy build the column assertion fails, and the
+        // RENDER_OUT frame of the wrapped two-column rail is exactly the "before" screenshot.
+        var dir = Environment.GetEnvironmentVariable("RENDER_OUT");
+        if (!string.IsNullOrWhiteSpace(dir))
+        {
+            Directory.CreateDirectory(dir);
+            using var frame = window.CaptureRenderedFrame()!;
+            frame.SavePng(Path.Combine(dir, "module-tabs-scrolling.png"));
+        }
+
+        var tabs = ((IncidentWorkspaceView)window.Content!).GetControl<TabControl>("ModuleTabs");
+        var tabItems = tabs.GetVisualDescendants().OfType<TabItem>().ToArray();
+
+        // Nothing lost: every declared rail tab is still realized.
+        Assert.Equal(10, tabItems.Length);
+
+        // One column, not side-by-side columns: every tab shares the same horizontal origin.
+        var columns = tabItems
+            .Select(t => t.TranslatePoint(new Point(0, 0), tabs)!.Value.X)
+            .Distinct()
+            .ToArray();
+        Assert.True(columns.Length == 1,
+            $"nav rail wrapped into {columns.Length} columns at x=" +
+            string.Join(", ", columns.Select(c => c.ToString("F0"))) +
+            " -- it must overflow into a scrollbar instead.");
+
+        // The overflow lands in a ScrollViewer, not silent clipping.
+        var strip = tabs.GetVisualDescendants().OfType<ItemsPresenter>()
+            .First(p => p.Name == "PART_ItemsPresenter");
+        var scroll = strip.GetVisualAncestors().OfType<ScrollViewer>().First();
+        Assert.True(scroll.Extent.Height > scroll.Viewport.Height,
+            $"rail is {scroll.Extent.Height:F0}px tall in a {scroll.Viewport.Height:F0}px viewport -- no overflow to scroll.");
+    }
+}