Empty undo actions list
This function releases any memory used to store an undo history, but still leaves undo functionality enabled for future operations. Therefore, if you are sure that you do not wish to undo previous operations you can reduce memory usage.
To run this function: Choose Edit > Undo properties > Empty undo actions list, or...
Undo information for all layers is purged.
A confirmation form is displayed so that the undo history is not inadvertently cleared.
All current undo and redo information for all layers is discarded.