Goal: Minimize memory and computational overhead drawing in-progress render result during offline rendering, similar to the viewport render.

Display Pass

There is one active display pass, which is communicated via Python API, similar to the active display pass for interactive rendering.

The render engine (i.e. Cycles) hold the full-frame pixel data of the display pass, preferably in the half-float data format.
When user changes display pass it is up to the render engine to load pass pixels from off-memory tile storage (i.e. Cycles will read pass pixels from the partial EXR files).

RenderEngine

RenderEngine has a draw callback (a complementary to view_draw()). This call takes care of drawing a GL texture which contains pixels of the display pass.

The drawing happens in the pixel space similar to the viewport. Draw manager takes care of setting up matrix stack which makes (possibly cropper) render to follow zoom and pan from the image space.

Draw manager

Draw manager is to use an external_engine to draw render result during rendering when the engine has the draw callback.
Once rendering is finished the regular RenderResult rendering is to be used.

Known limitations

One of the immediate limitation is inability to query pixel value and depth during rendering. It might be considered to be implemented in the future, but for an initial implementation it is a sacrifice to be made.