The generic rule

Changes that do not make data change on the board are generally not undoable.

Examples of what is undoable

The ones marked with * may not yet be undoable but should be.

• creating or deleting drawing primitives on the board
• changing geometry of drawing primitives on the board
• changing metadata of drawing primitives on the board
• changing board metadata (e.g. size, name)
• routing style changes (*)
• layer stack changes (*)

Examples of what is not undoable

• changes made to paste buffers: those are not board data
• crosshair coordinate, style
• config setting changes, including but not limited to:
  • grid settings
  • GUI related settings (e.g. compact layout)
• fonts loaded/unloaded to/from the board

Cases that may look strange

• selection changes are undoable because selection is stored in object flags which is covered by "metadata of drawing primitives".
• some of the config is saved in the board (the design role) - changes in such config will mark the board changed and saving the board may cause a diff, but they are not undoable because config changes are generally not undoable