From: https://community.aseprite.org/t/add-preference-for-dragged-selection-to-respect-snap-to-grid-setting/12884

If we move the selection and the Snap to Grid option is enabled, we should move the selection with snap to grid by default (and Alt modifier doing the inverse, disabling the snap to grid).

At the moment the only way to move the selection with snap to grid is pressing Alt.