Steps to reproduce.
1) Open large image, image is automatically zommed out to fit window.
2) Adjust window size so that the width:height ratio closely mimics that of the image, i.e. very little whitespace left around the image. (This step is important.)
3) Click on "Zoom in" so the scroll bars appear.
4) Click on "Fit image to window size".
5) BUG: scroll bars still appear, so a small strip along the bottom and right of the image is covered.
Analysis. The following is my guess only.
- When we zoomed in, the scroll bars caused the viewable area size to be smaller than the window size.
- When we fit to window, image is resampled to window size, but since this exceeds the current viewable area, the scroll bars remain.
- Resulting in a non-fit.
- I have no idea how it works. Sorry.