The pop-up loupe, has the potential to be a transformational feature. It has
the potential (when properly designed) to fully transform the way most people work with GIMP in work-macroscopic/change-microscopic situations, that goes way beyond setting selections pixel-precise.

The pop-up loupe is meant to be used combination with any odd toolbox tool, press a certain keyboard key and the loupe pops up for adjusting microscopic things, release the key and it is gone.

The guiding principle is that the enlarged view is both very lose and out of the way of the mouse pointer, similar to the way that is shown in this image:

http://www.creativepro.com/img/story/20051219_fg5.jpg

The temporary loupe shall have its own zoom level and also have an option to work absolute (to the image data) or relative (to the zoom level in the actual image window). This means you can set it to display the image data at 400%, or 400% of the window level (window is at 33%, result in the loupe is 132%). And all this of course not far away in the preferences, but preferably in a HUD (heads-up display) when the loupe is there.

The dark edges of the loupe shall be semi-transparant to get the best balance between function (tying the mouse pointer and loupe view together) and obstruction. This requires cairo.

The mouse pointer shall also be displayed in the loupe view, enlarged for the
paint tools. The speed at which the mouse pointer traverses pixels because of mouse movements shall be the natural speed of the loupe view, to afford working
microscopically.