Here is a suggestion for FancyBox, if it is possible ...
Attached are a couple of screenshots on Safari, on an iPhone6, of a FancyBox gallery.
The first shows the actual slide, i.e., as it appears. The image is centered in the available viewport space, so that it doesn't overlap the menu icons at the top. An area equivalent to that top menu-icon space is unused at the bottom, below the image.
The second screenshot shows my suggestion. It is a pinch-zoom of the image, and then lowered slightly. That is, the image itself is no longer centered, because there is no longer an unused area at the bottom of the viewport. This allows the image itself to be larger, while still maintaining its aspect ratio.
This change is similar to what already happens in a desktop browser with a FancyBox gallery. That is, if the thumbnails are shown along the bottom of a desktop viewport, the image is a certain size (and not centered, top to bottom). Once the bottom thumbnails are closed (by clicking on the corresponding menu icon), the image is expanded, because there is more room available for it.
Of course, I may be missing something here, there may be some reason why this is not feasible on a phone, that sort of thing.
Set new check-box 'Use whole screen for slide image if screen height <' on the Slide page / Layout tab.
If you enter a value for the minimum screen height after that check-mark, it is only applied for small devices with a screen height below the given value. So I see it only on my iPhone if I enter a minimum screen height of 400 px.