Single-entity View does not scroll when the entity is taller than the viewport (unbounded vertical viewport + ClipToBounds chain)

[JoshuaRowePhantom]

Summary

When a single entity's "View" (the EntityWorkspaceTabViewModel workspace tab) is taller than the visible pane, the view cannot be scrolled — the lower portion of the entity is clipped and unreachable, and the mouse wheel does nothing. The tab's DataTemplate wraps the card in a ScrollViewer with VerticalScrollBarVisibility="Auto", but the content chain never establishes a bounded vertical viewport, so at runtime Extent.Height == Viewport.Height and no scrollbar ever engages. Two nested ClipToBounds="True" layers then silently clip the overflow instead of surfacing it.

Root Cause

The single-entity view is rendered by the DataTemplate for EntityWorkspaceTabViewModel in features\Phantom.Workspaces\Templates\WorkspaceDataTemplates.axaml:35-55 (added by commit c58069a6, "Fix #1066"):

<DataTemplate DataType="vm:EntityWorkspaceTabViewModel">
    <ScrollViewer Classes="entity-card-single"
                  HorizontalScrollBarVisibility="Auto"
                  VerticalScrollBarVisibility="Auto"
                  AllowAutoHide="False">
        <ContentControl Classes="entity-card-shell entity-card-single-host"
                        DataContext="{Binding EntityCardNode}"
                        Content="{Binding}"
                        HorizontalAlignment="Center"
                        VerticalAlignment="Top"
                        Margin="0,12,0,0"
                        MinWidth="160">
            <ContentControl.MaxWidth> ... </ContentControl.MaxWidth>
        </ContentControl>
    </ScrollViewer>
</DataTemplate>

The visual tree inside the tab is:

DockControl (document host — MainWindow.axaml:253-266, no outer scroll wrapper)
 └─ ScrollViewer (entity-card-single, V=Auto, H=Auto, AllowAutoHide=False)
     └─ ContentControl (entity-card-shell entity-card-single-host, VAlign=Top, HAlign=Center, no Height/MaxHeight)
         └─ ControlTemplate root: StackPanel               (SharedStyles.axaml:207)
             └─ Border (entity-card-shell-border, ClipToBounds=True)   (SharedStyles.axaml:210)
                 └─ ContentPresenter
                     └─ EntityCardControl root: StackPanel (ClipToBounds=True)  (EntityCardControl.axaml:10-14)
                         └─ header / field editors / external card / raw JSON / status

The failure is a measure-constraint / height-propagation problem, not a disabled scrollbar:

1. The inner ContentControl (WorkspaceDataTemplates.axaml:40) uses VerticalAlignment="Top" and sets no Height/MaxHeight, so its DesiredSize.Height is the full (unbounded-upward) height the card wants.
2. A ScrollViewer only shows a vertical scrollbar when its arranged Bounds.Height is less than the content's DesiredSize.Height. When the dock document does not impose a finite height on the ScrollViewer — or when an ancestor measures the Y axis with PositiveInfinity (an Auto-height slot / StackPanel-like measure / nested outer scroller) — the ScrollViewer's Viewport.Height equals its Extent.Height, so no scrollbar appears and the wheel has no effect. This matches the reported symptom exactly.
3. ClipToBounds="True" on both the shell Border (features\Phantom.Workspaces.Gui.Shared\Styles\SharedStyles.axaml:210) and the EntityCardControl root StackPanel (features\Phantom.Workspaces\Controls\EntityCardControl.axaml:13) means the overflow of the tall card is silently clipped rather than made visible — hiding the fact that layout produced an unbounded-height chain.

There is no VerticalScrollBarVisibility="Disabled" and no MaxHeight anywhere in the chain; the string VerticalScrollBarVisibility="Auto" is present but ineffective because the viewport is never bounded relative to content.

Host context. The tab body is hosted directly in Phantom.Dock.Avalonia's DockControl (features\Phantom.Workspaces\MainWindow.axaml:253-266, AutoCreateDataTemplates="False"), with no app-level ScrollViewer around the document. The EntityWorkspaceTabViewModel DataTemplate is the sole layout for the tab body, so responsibility for making the over-tall entity scrollable lives entirely in that template/shell chrome — confirming the bug is here, not in outer chrome.

Why it slipped through. Commit c58069a6 (Fix #1066) added only the horizontally-centred, width-capped chrome and covered it with XAML text-scan tests (features\Phantom.Workspaces.Agent.Gui.Tests\MainWindowAxamlTests.cs:435-451) that assert the literal string VerticalScrollBarVisibility="Auto" but never realize the visual tree, so they cannot detect that Extent/Viewport never diverge at runtime.

Affected Files

File	Contribution
features\Phantom.Workspaces\Templates\WorkspaceDataTemplates.axaml	EntityWorkspaceTabViewModel DataTemplate (:35-55); ScrollViewer :36; ContentControl VerticalAlignment="Top", no Height/MaxHeight :40-46
features\Phantom.Workspaces.Gui.Shared\Styles\SharedStyles.axaml	entity-card-shell template root StackPanel (:207); shell Border ClipToBounds="True" (:210)
features\Phantom.Workspaces\Controls\EntityCardControl.axaml	Card root StackPanel ClipToBounds="True" (:10-14)
features\Phantom.Workspaces\MainWindow.axaml	Dock host, no outer scroll wrapper (:253-266)
features\Phantom.Workspaces.Agent.Gui.Tests\MainWindowAxamlTests.cs	XAML text-scan tests that can't catch the runtime failure (:435-451)

Design / Fix

The essential change: make the ScrollViewer arrange its content against a finite vertical viewport, and stop clipping overflow in the scrollable chain.

Option A — most surgical. In WorkspaceDataTemplates.axaml:40-46, remove VerticalAlignment="Top" from the ContentControl (move the 12px top gap onto a wrapping panel/margin that does not force top-alignment), so the ScrollViewer compares its finite arranged height against real content height. Verify no ancestor imposes an infinite vertical measure on the ScrollViewer.

Option B — belt-and-braces. Wrap the ContentControl in a Grid (single */Auto row) inside the ScrollViewer and remove ClipToBounds="True" from the shell Border (SharedStyles.axaml:210) and the EntityCardControl root StackPanel (EntityCardControl.axaml:13) so overflow is scrollable/visible rather than silently clipped:

<ScrollViewer Classes="entity-card-single"
              HorizontalScrollBarVisibility="Auto"
              VerticalScrollBarVisibility="Auto"
              AllowAutoHide="False">
    <Grid RowDefinitions="Auto">
        <ContentControl Grid.Row="0"
                        Classes="entity-card-shell entity-card-single-host"
                        DataContext="{Binding EntityCardNode}"
                        Content="{Binding}"
                        HorizontalAlignment="Center"
                        Margin="0,12,0,0"
                        MinWidth="160">
            ...
        </ContentControl>
    </Grid>
</ScrollViewer>

Whichever option is chosen, confirm the dock parent arranges the ScrollViewer into a finite-height slot, and preserve the #1066 width-capping / horizontal-centering behaviour and the #1064 two-regime horizontal scroll handling.

Expected Tests

Existing coverage is XAML text-scans only (MainWindowAxamlTests.cs:391-451, SharedStylesTests.cs, EntityWorkspaceTabViewModelTests.cs — none realize an over-tall card and assert scroll behaviour). Add headless layout tests (using HeadlessUnitTestSession per project conventions), in a new SingleEntityViewScrollTests in features\Phantom.Workspaces.Agent.Gui.Tests\:

Test Name	Class	What It Verifies
SingleEntityView_WhenContentTallerThanViewport_ScrollViewerExtentExceedsViewport	SingleEntityViewScrollTests	With an over-tall entity, the ScrollViewer's Extent.Height > Viewport.Height
SingleEntityView_WhenContentTallerThanViewport_VerticalScrollBarIsVisible	SingleEntityViewScrollTests	The vertical scrollbar becomes visible
SingleEntityView_WhenContentTallerThanViewport_MouseWheelChangesVerticalOffset	SingleEntityViewScrollTests	A wheel gesture changes the vertical Offset
SingleEntityView_WhenContentFitsViewport_VerticalScrollBarStaysHidden	SingleEntityViewScrollTests	Short content does not show a scrollbar
SingleEntityView_EntityCardShellChrome_DoesNotClipOverflowVertically	SingleEntityViewScrollTests	Guards against re-introducing ClipToBounds="True" in the scrollable chain

If full headless realization of the templated shell proves infeasible, an intermediate guard — asserting the DataTemplate's visual tree does not put VerticalAlignment="Top" on the immediate child of the vertically-scrolling ScrollViewer — at least prevents regression of Option A.