Table of Contents

OpenTK FAQ

Common problems and fixes when using DrawnUI with OpenTK.

White flash appears before the first frame

The window briefly shows white before content appears.

Cause: Window becomes visible before the first GL frame is rendered.

Fix for DrawnUiWindow: Handled automatically — the base class sets StartVisible = false internally and sets IsVisible = true after the first SwapBuffers().

Fix for CanvasHost / custom GameWindow: Pass StartVisible = false in NativeWindowSettings, track a _firstFrameDone bool, and reveal the window after the first SwapBuffers():

private bool _firstFrameDone;

// In NativeWindowSettings:
StartVisible = false,

// In OnRenderFrame, after SwapBuffers():
SwapBuffers();
if (!_firstFrameDone)
{
    _firstFrameDone = true;
    IsVisible = true;
}

3D scene is invisible — geometry doesn't render

Scene is black or geometry is missing despite correct draw calls.

Cause: Skia leaves GL state dirty after rendering. Without restoring it, 3D draw calls fail silently.

State What Skia does Effect if not restored
GL_VIEWPORT Sets its own viewport Vertices map off-screen — invisible
GL_STENCIL_TEST Enables with its own ref/func All fragments fail stencil → nothing draws
glDepthMask Sets to false Depth clear has no effect; depth test corrupts
Color mask May be partial Some color channels don't write

Fix: Restore all four states before every raw GL draw, and call ResetGrContext() before Skia renders:

// Before your GL draw:
GL.Viewport(0, 0, ClientSize.X, ClientSize.Y);
GL.Disable(EnableCap.StencilTest);
GL.DepthMask(true);
GL.ColorMask(true, true, true, true);

// ... your scene ...
GL.Finish();

// Before DrawnUI renders:
_host.ResetGrContext();
_host.Render();

See Window Patterns for the full frame order.

3D scene disappears when the DrawnUI overlay renders

Background goes white or the GL scene is overwritten by Skia's clear.

Cause: Canvas.RenderingMode is not AcceleratedRetained, or BackgroundColor is opaque.

Fix:

new Canvas
{
    BackgroundColor = Colors.Transparent,
    RenderingMode = RenderingModeType.AcceleratedRetained,
    ...
}

AcceleratedRetained skips Skia's internal canvas.Clear() so the GL framebuffer is preserved when the overlay composites on top. Your own GL.Clear(...) still runs every frame before your scene draw — that resets the buffer without involving Skia, so there is no alpha accumulation.

This mode is required for any overlay that uses SkiaBackdrop — without it, SkiaBackdrop has no framebuffer content to sample and appears blank.

SkiaEditor loses focus on mouse release

Clicking inside SkiaEditor focuses it on mouse down, but focus is lost immediately when the mouse button is released.

Cause: Canvas.ProcessNetGestures runs two passes for the Up event. The editor correctly claims Up in the first pass (HadInput). In the second pass, the gesture walks the control tree; the editor returns null (already counted via SignalInput dedup) and the traversal falls through to the next sibling — typically SkiaBackdrop, which unconditionally returns this from ProcessGestures. The canvas sees consumed (SkiaBackdrop) != FocusedChild (SkiaEditor) and clears the editor's focus.

Status: Fixed in Canvas.Gestures.Net.cs, Canvas.cs (MAUI), and Canvas.Gestures.Blazor.cs. The consumed != FocusedChild focus-reassignment guard no longer fires on Up events — focus ownership is established on Down only.

If you write a custom control that sits behind an editor in z-order: do not return this unconditionally from ProcessGestures. Return null for Up when the control did not capture the gesture on Down.

Mouse clicks or drags don't reach the DrawnUI overlay

Controls don't respond to mouse input.

Cause: Input events not forwarded to the canvas (CanvasHost only — DrawnUiWindow wires this automatically).

Fix: Override all four mouse events in your GameWindow:

protected override void OnMouseDown(MouseButtonEventArgs e)
{
    base.OnMouseDown(e);
    if (e.Button == MouseButton.Left)
        _canvas.HandleDesktopPointerDown(MousePosition.X, MousePosition.Y, ClientSize.X, ClientSize.Y);
}

protected override void OnMouseMove(MouseMoveEventArgs e)
{
    base.OnMouseMove(e);
    _canvas.HandleDesktopPointerMove(MousePosition.X, MousePosition.Y,
        MouseState.IsButtonDown(MouseButton.Left), ClientSize.X, ClientSize.Y);
}

protected override void OnMouseUp(MouseButtonEventArgs e)
{
    base.OnMouseUp(e);
    if (e.Button == MouseButton.Left)
        _canvas.HandleDesktopPointerUp(MousePosition.X, MousePosition.Y, ClientSize.X, ClientSize.Y);
}

protected override void OnTextInput(TextInputEventArgs e)
{
    base.OnTextInput(e);
    _canvas.HandleDesktopTextInput(e.AsString);
}

SkiaEditor doesn't type — keyboard input is lost

Clicking the editor focuses it but keys produce no text and navigation keys don't work.

Cause: Editor-key events not forwarded (CanvasHost only — DrawnUiWindow handles this automatically).

Fix: Override OnKeyDown and dispatch to canvas editor helpers:

protected override void OnKeyDown(KeyboardKeyEventArgs e)
{
    base.OnKeyDown(e);
    var shift = KeyboardState.IsKeyDown(Keys.LeftShift) || KeyboardState.IsKeyDown(Keys.RightShift);
    var ctrl  = KeyboardState.IsKeyDown(Keys.LeftControl) || KeyboardState.IsKeyDown(Keys.RightControl);
    switch (e.Key)
    {
        case Keys.Backspace: _canvas.DesktopEditorBackspace(); break;
        case Keys.Delete:    _canvas.DesktopEditorDelete(); break;
        case Keys.Enter:     _canvas.DesktopEditorEnter(); break;
        case Keys.Left:      _canvas.DesktopEditorMoveCursor(-1, shift); break;
        case Keys.Right:     _canvas.DesktopEditorMoveCursor(1, shift); break;
        case Keys.Home:      _canvas.DesktopEditorMoveToStart(shift); break;
        case Keys.End:       _canvas.DesktopEditorMoveToEnd(shift); break;
        case Keys.A when ctrl: _canvas.DesktopEditorSelectAll(); break;
        case Keys.Tab:       _canvas.HandleDesktopTextInput("    "); break;
    }
}

OnTextInput must also be forwarded (see Mouse clicks don't reach the overlay above).

SkiaBackdrop / glass effect appears blank or white

SkiaBackdrop renders as a solid white or transparent rectangle instead of blurring the scene behind it.

Cause: RenderingMode is not AcceleratedRetained. Skia clears the canvas before drawing, so SkiaBackdrop has no framebuffer pixels to sample.

Fix: Set RenderingMode = RenderingModeType.AcceleratedRetained and BackgroundColor = Colors.Transparent on the Canvas. Both are required for backdrop effects in an overlay.

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