Made steady progress this morning. The flickering was the thumbnails never being reused (the CALayer returned by GetHandle() changes on GL but was constant on Vulkan). At one point there was 128 thumbnails when there should have been 2. Fixing that makes performance much nicer and removes most of the flickering.
Overall it is working well. One thing not quite right is the main texture window/pane resize seems to be required to trigger things properly ie. resize on the pixel context pane with the thumbnails not being picked up as a resize event properly. I need to debug that.
Still a bit of flickering in the video but the UI feels much nicer to use today than it did yesterday (even in a debug build)
Jake A Turner Blog 