To quote from the V2.8 on-line documentation for the New View menu command:

"Any change, other than viewing changes, which you make in one window also appear in the other displays which show the same image. The new views are not separate image files; they are simply different aspects of the same image. You might use multiple views, for example, if you were working on individual pixels at a high zoom factor. You could then see the effects your changes would have on the image at a normal size."

At the moment I think that this really only applies to the airbrush and the colour tools (Levels, Curves, Brightness etc).

The paintbrush type tools (pencil, eraser etc) do reflect the changes but only when the mouse button is released (whereas the airbrush shows the line in all of the views as it is being drawn).

More importantly the additional views are not updated during transform operations. In fact with the later versions of 2.9.5, in which the active layer is made transparent when the transform tool is activated, the additional views of an image with only one layer all change to the chequerboard pattern - which looks a bit odd and may be the cause of a number of questions/bug reports.

Displaying the transform preview in the additional views would conform to the aim stated in the documentation of allowing views of the changes at different magnifications to that used for the active view. In particular it would allow a higher magnification to be used for an additional view to see the effects of the Unified Transform tool - where you can't zoom the active view otherwise the control handles are not visible and all you can do is rotate or move the image. Obviously this isn't a problem for the perspective, shear etc tools as the actions still accessible when zoomed in.

Changing the paint tools so that they reflect the changes 'in real time' is trivial - in functions gimp_paint_tool_button_press() and gimp_paint_tool_motion() use the existing gimp_display_flush_now (display) call when the image has only one display (this makes the program more responsive), otherwise use gimp_image_flush(image).

Unfortunately, however, I don't have the experience to suggest changes to make the transform preview appear in the additional views - this being by far the more useful change.

BTW, this would have been more suitable for the gimp-developer mailing list.

I'm not a member of the gimp-developer mailing list - perhaps I should join - can I join?

There is a speed penalty so it might be as well to add an option to Preferences to allow the user to turn this feature off. But even then the behaviour of the views would not be consistent as the airbrush would (unless modified from its current behaviour) still update the additional views as it is used.

Whilst it may be helpful to modify the paint tools in this way my aim was to modify the transform tools so that the preview appears in all views - thereby adding to the usability of the Unified Transform tool.

If someone could give me any pointers on how this might be achieved I would be pleased to look further at it.

I have only just started looking at the gimp sources and trying to build it on Windows (with some success but I still have a few things to sort out) so I'm not familiar with the inner workings of the program. I had assumed that there would be a single canvas for each image and that all of the views for the image were simply apertures on to this single canvas. So when the canvas had been altered it was simply a case of redisplaying the appropriate portions in the one or more views. Given the speed at which the image viewer I use can switch between two full screen images I was expecting that the generally smaller additional views would not take long to refresh.

On a separate matter - when compiling/building gegl and gimp there are numerous warnings displayed (some of these are, I think, only output when building on/for Windows). I could work at eliminating these warnings to allow a clean build if such changes would be welcome. Any warnings that then appear as the result of future changes will then stand out and can be addressed or suppressed as appropriate.