keepViewportMainAxisSize property
The flag to keep main axis size of each photo page to origin size (which is the same as default identical viewport fraction), defaults to false.
Note that if fractionWidthFactor or fractionHeightFactor set to null or non-positive number, the actual fractional page factor will be depended by keepViewportMainAxisSize and PageController.viewportFraction.
Implementation
final bool keepViewportMainAxisSize;