Press Command+Z to undo and Shift+Command+Z to redo. Every operation is named, so the Edit menu always tells you exactly what will happen—“Undo Gradient Fill”, “Undo Merge Down”, “Undo Cut Selection”, and so on.
Each undo step restores a complete document snapshot—layers, selection, palette, drawing settings, and the reference image together—so even multi-layer operations come back exactly as they were. Dottie keeps the most recent 50 steps.
Undo also covers view and document-structure changes that affect how the canvas is interpreted: switching the view mode (Normal, Tileset, or Animation), changing the tileset column count, toggling Wrap Tiles, adjusting the pixel aspect ratio, and running Split into Tiles or Merge Tiles into Canvas. All of these land as named undo steps and can be reversed with Command+Z.
Other named steps you may notice in the Edit menu:
Import Image — File > Import (and sprite-sheet imports) as one step, including floating selection or new layers, palette colours, and any canvas resize from the import. A failed import does not change the document.
Reference image — Add, Remove, Fit, Show/Hide, and inspector edits to opacity, scale, and offset (slider drags coalesce into one step).
Dither pattern — selecting or clearing a custom pattern; undoing other edits also restores the pattern that was active at that step.
Versions: because Dottie uses the standard macOS document system, File > Revert To lets you browse and restore previously saved versions of the document.
Live colour edits: editing a palette colour previews across the whole document but lands as a single undo step when you close the colour picker.
Scripting: undo and redo are available to AppleScript.