DockPanel.Dock="Bottom" content invisible at 125% display scaling (100% works)

[ajoh99]

Describe the bug

At 125% display scaling on Windows, DockPanel.Dock="Bottom" content is not rendered at all — the sibling Fill element (the last, un-docked child) paints over the entire remaining area with no visible trace of the docked content, and no error, warning, or exception of any kind.

At 100% scaling, the identical XAML renders correctly.

This reproduces with the two most basic controls Avalonia has (TextBlock + Border), with no bindings, styles, or custom controls involved, so it isn't specific to any particular app.

Reproduction steps

1. Create a new Avalonia app from the default template (dotnet new avalonia.app).
2. Set the Windows display scaling to 125% (Settings → System → Display → Scale).
3. Replace MainWindow.axaml content with:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="DockBottomRepro.MainWindow"
        Title="DockBottomRepro">

    <DockPanel>
        <TextBlock DockPanel.Dock="Bottom" Text="BOTTOM TEXT" Background="Yellow" FontSize="24" />
        <Border Background="LightBlue" />
    </DockPanel>

</Window>

4. Run the app.

Expected: a yellow "BOTTOM TEXT" strip at the bottom of the window, light blue everywhere else (matches the behavior at 100% scaling, and matches WPF's DockPanel behavior).

Actual: the window is solid light blue — the bottom-docked TextBlock never appears. No exception, no binding error, no XAML warning. Confirmed via a real desktop screen capture (not a PrintWindow/GDI capture, to rule out a screenshot-tool artifact) with the window genuinely in the foreground.

Setting DockPanel.Dock="Top" on the same TextBlock (with the same Border as the trailing/fill child) works correctly at 125% scaling — only Bottom is affected in my testing (I did not exhaustively test Left/Right).

What I've ruled out

I hit this in a real app (a DockPanel inside a UserControl inside a Window, with real bindings) and spent considerable effort isolating the actual cause before landing on display scaling. Along the way I ruled out:

• Avalonia version: reproduces identically on 11.3.20 (latest stable 11.x) and 12.1.1 (latest stable 12.x) — not a v12-specific regression.
• Rendering backend: reproduces identically with Win32PlatformOptions.RenderingMode = [Win32RenderingMode.Software] forced — not a GPU/driver-specific compositing issue. (Machine has a hybrid Intel UHD 630 / NVIDIA GTX 1650 Max-Q Optimus setup, in case that's independently relevant to anyone else's repro.)
• Client-side decorations: reproduces identically with WindowDecorations="Full" + ExtendClientAreaToDecorationsHint="False" explicitly set (i.e. not specific to the new Avalonia 12 app-drawn decorations path).
• AvaloniaUI.DiagnosticsSupport / .WithDeveloperTools(): reproduces identically with that hook removed entirely.
• UseLayoutRounding="False" on the Window (the fix for Layout precision error in Avalonia 11 #8866, a different fractional-DPI layout bug that also manifests specifically at 125%): does not fix this — so this looks like a distinct issue from Layout precision error in Avalonia 11 #8866, just triggered by the same category of non-integer scale factor.

The one variable that reliably flips the behavior, tested by switching the actual Windows display scaling setting (not a manifest/registry per-app DPI override) and relaunching each time:

Scaling	Result
100%	✅ Bottom-docked content renders correctly
125%	❌ Bottom-docked content invisible

Environment

• Avalonia: 11.3.20 and 12.1.1 (both tested, same result)
• OS: Windows 10 Home, 10.0.19045
• .NET SDK: 10.0.400, target framework net10.0
• GPU: Intel(R) UHD Graphics 630 (driver 26.20.100.7870) + NVIDIA GeForce GTX 1650 with Max-Q Design (driver 27.21.14.5749) — hybrid/Optimus laptop
• Display scaling: 125% (bug present) vs 100% (bug absent), single monitor, no mixed-DPI setup involved

Possibly related

I found some older/adjacent reports that may share a root cause or at least the same general area of the layout engine, though none matches this exactly:

• Panel does not take all remaining space #4628 (closed) — a comment from a different user describes the same "DockPanel Top/Bottom docks overlap in newer versions, Grid RowDefinitions="Auto,*,Auto" works instead" symptom, but in my own testing that Grid pattern (Auto-row-after-*-row) also failed to render the trailing content at 125% scaling — so if it's the same bug, the reported workaround isn't reliable either, at least not on this setup.
• DockPanel Docking not completly working #348, DockPanel overlay elements #2639 — older DockPanel dock-order/overlay rendering bugs, possibly unrelated but in the same area of the codebase.
• Layout precision error in Avalonia 11 #8866 — the closest DPI-specific precedent I found (also 125%-specific, also fine at 100%), but its confirmed root cause (Border not accounting for its own layout rounding when arranging an opaque child) and its fix (UseLayoutRounding="False") do not apply here — I tested it directly and it made no difference.

Happy to provide a runnable repro project if useful.

[emmauss]

Please provide a sample project