CompletionWindow causes application crash (InvalidOperationException in GlRenderTarget) when all items are filtered out

[DamirLisak]

Description

When using CompletionWindow with filtering enabled, typing a character that reduces the completion list to zero matching items causes the application to crash on OpenGL/Skia backends. The application becomes unresponsive and on Linux can only be terminated by killing the process.

Steps to Reproduce

1. Open a document with XML syntax highlighting.
2. Type < to trigger the completion window.
3. Type ! to filter the list down to a single item (e.g. "Comment").
4. Type . (or any other character that results in zero matching items).

Expected: The completion window closes gracefully.
Actual: The application crashes with an unhandled InvalidOperationException.

Exception

System.InvalidOperationException: Can't create drawing context for surface with 241, 0 size and 1 scaling
   at Avalonia.Skia.GlRenderTarget.BeginRenderingSessionCore(Nullable`1 expectedSize)

Surface size at time of crash: Width = 241, Height = 0.

Call Stack

Avalonia.Skia.GlRenderTarget.BeginRenderingSessionCore(Nullable`1 expectedSize)
Avalonia.Skia.GlRenderTarget.BeginRenderingSession(PixelSize size)
Avalonia.Skia.SkiaGpuRenderTarget.CreateDrawingContextCore(...)
Avalonia.Skia.SkiaGpuRenderTarget.CreateDrawingContext(...)
Avalonia.Platform.RenderTargetExtensions.CreateDrawingContextWithProperties(...)
Avalonia.Rendering.Composition.Server.ServerCompositionTarget.Render()
Avalonia.Rendering.Composition.Server.ServerCompositor.RenderCore(bool catchExceptions)
Avalonia.Rendering.Composition.Server.ServerCompositor.RenderReentrancySafe(bool catchExceptions)
Avalonia.Rendering.Composition.Server.ServerCompositor.Render(bool catchExceptions)
Avalonia.Rendering.Composition.Server.ServerCompositor.Render()
Avalonia.Rendering.RenderLoop.TimerTick(TimeSpan time)
Avalonia.Rendering.SleepLoopRenderTimer.LoopProc()

Root Cause

Two cooperating bugs:

Bug 1 — CompletionWindowBase.Show() resets MinHeight to 0:

public void Show()
{
    UpdatePosition();
    Open();
    Height = double.NaN;
    MinHeight = 0;  // ← overwrites MinHeight = 15 set in CompletionWindow constructor
}

CompletionWindow's constructor sets MinHeight = 15 to prevent a zero-height popup. However, Show() immediately overwrites this with MinHeight = 0, removing the safeguard.

Bug 2 — Zero matches hides the list instead of closing the window:

In CaretPositionChanged, when the filtered list becomes empty, the code only hides the CompletionList child control:

if (CompletionList.ListBox.ItemCount == 0) CompletionList.IsVisible = false;

The popup itself stays open (IsOpen = true) but its only child is invisible, so Avalonia lays it out at height = 0. Because MinHeight was reset to 0, there is no constraint preventing this.

The renderer then attempts to create a drawing context for a surface of size 241×0, which hits this guard in GlRenderTarget.BeginRenderingSessionCore:

if (size.Width <= 0 || size.Height <= 0 || scaling < 0)
    throw new InvalidOperationException(
        $"Can't create drawing context for surface with {size} size and {scaling} scaling");

This exception does not match the GPU context-loss filter in ServerCompositor.RenderCore, so it propagates unhandled and crashes the render thread.

Fix

See this commit:
iplus-framework@7f4faaf

CompletionWindowBase.cs — Show()

Removed MinHeight = 0. This line overwrote the MinHeight = 15 set in CompletionWindow's constructor, allowing the popup to be laid out at zero height by Avalonia's layout engine when its child was hidden.

// Before
public void Show()
{
    UpdatePosition();
    Open();
    Height = double.NaN;
    MinHeight = 0;  // removed
}

// After
public void Show()
{
    UpdatePosition();
    Open();
    Height = double.NaN;
}

CompletionWindow.cs — CaretPositionChanged()

When ItemCount == 0 and CloseAutomatically is true, the window now calls Hide() instead of setting CompletionList.IsVisible = false. An open popup with no visible content and MinHeight = 0 results in a zero-height surface being submitted to the renderer, which throws an unhandled InvalidOperationException on OpenGL/Skia backends and deadlocks the render thread. Closing the window is also the correct UX behavior — if nothing matches, there is nothing to show.

// Before
if (CompletionList.ListBox.ItemCount == 0) CompletionList.IsVisible = false;
else CompletionList.IsVisible = true;

// After
if (CompletionList.ListBox.ItemCount == 0)
{
    if (CloseAutomatically) { Hide(); return; }
    CompletionList.IsVisible = false;
}
else CompletionList.IsVisible = true;

The IsVisible = false path is preserved for the CloseAutomatically = false case. The restored MinHeight = 15 provides an additional safety net even in that path.

For the sake of simplicity, I would be grateful if the code could be adopted directly without the cumbersome process of a pull request.

[DamirLisak]

@maxkatz6 since I saw that you have been active in the last few days to prepare Release 12, I would be happy if you would include this pull request 🙏.
#569