Only occurs if file was empty (or contained only blank layers) before cels were added.
If cels were added in order from lower to higher frame numbers, each cel except the first is moved to the first non-blank frame above it (the first cel is removed).
If added in mixed order, some are removed and the rest swapped (can't discern pattern).
Only occurs if file was empty (or contained only blank layers) before cels were added.
If cels were added in order from lower to higher frame numbers, each cel except the first is moved to the first non-blank frame above it (the first cel is removed).
If added in mixed order, some are removed and the rest swapped (can't discern pattern).
If added in reverse order, all are removed.