- it would be nice to be able to fine tune the marker position by hovering above the marker and using the mouse scroll-wheel to move the marker left/right (just like you can do on the boxes below the top window)

- maybe fine tunning the marker position could also be done by grabbing the marker and moving it left/right while you press the Alt (or ctrl) key. This could be a sort of 'divider' function that translates a mouse movement of 100 pixels to a value change of say only 10 samples. (a similar option is available in blender, allowing you to be more precise when you move objects around)