Game Boy transparency example.

Render the Game Boy window, with transparency, supporting arbitrary scrolling.

Render any rectangular section, with transparency.

Render arbitrary sections, with transparency.

It might be possible to properly display subsections of the Game Boy window by disabling it for all scanlines that don't need it.

This would enable any window Y cross-section, but since the entire window is rendered, such windows must be on the right side of the display.