[WpfGfx] RenderTargetBitmap can produce blank output in headless sessions

[etvorun]

[WpfGfx] RenderTargetBitmap can produce blank output in headless sessions

Summary

RenderTargetBitmap may produce blank images when an app runs without attached display devices.

Observed behavior

• Rendered bitmap output is blank in headless or temporarily display-detached environments.
• The issue is intermittent but can persist until display topology is restored.

Repro notes

1. Start a WPF app that uses RenderTargetBitmap.Render.
2. Run the app in a session where no display devices are currently available (for example, a disconnected remote environment).
3. Trigger bitmap rendering and inspect the result.

Impact

• Breaks scenarios that depend on off-screen bitmap generation.
• Affects remote/headless workflows and reconnect scenarios where image output must remain reliable.

Expected behavior

RenderTargetBitmap.Render should render valid bitmap content even when no display is attached for this UI-thread synchronous rendering path.

Actual behavior

Rendering can be skipped when display state is invalid, causing blank bitmap output.

Suspected root cause

The composition pipeline blocks rendering on invalid display state unless a compatibility switch is enabled globally. The synchronous UI-thread bitmap rendering path needs a targeted override.

Proposed fix direction

• Thread a canRenderWithoutDisplayDevices flag through Partition::Compose and CComposition::ProcessComposition.
• Pass true for the UI-thread synchronous compose path used by RenderTargetBitmap-style rendering.
• Pass false in the regular partition-thread rendering path to preserve existing default behavior.
• Keep compatibility-switch behavior intact as an additional opt-in path for general rendering.