22. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by a first electronic device with a display, the first electronic device including one or more user-interface devices for receiving editing inputs from a user of the first electronic device, cause the first electronic device to: maintain a directed acyclic graph to represent content collaboratively edited by the first electronic device and one or more second electronic devices of multiple collaborating devices, wherein the directed acyclic graph includes a plurality of nodes each representing a respective content object that is created or edited by one or more of the multiple collaborating devices, wherein at least a first node of the plurality of nodes represents a textual content object and at least a second node of the plurality of nodes represents a sketch content object, and wherein each node representing a corresponding sketch content object includes a respective command sequence used to create content of the corresponding sketch content object; during a respective synchronization period, receive one or more editing inputs from one or more devices of the multiple collaborating devices; modify the directed acyclic graph based on relationships between the editing inputs and existing content objects embodied in the directed acyclic graph; determine whether the one or more editing inputs modifies an existing sketch content object represented in the directed acyclic graph; and in accordance with a determination that a first editing input of the one or more editing inputs modifies a first existing sketch content object represented by a first command sequence in a first respective node in the directed acyclic graph, update the first command sequence in the first respective node by merging each individual drawing command included in the first editing input with the first command sequence to produce, in the first respective node, an updated first command sequence of two or more commands that represents the first existing sketch content object as modified by the first editing input.
22. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by a first electronic device with a display, the first electronic device including one or more user-interface devices for receiving editing inputs from a user of the first electronic device, cause the first electronic device to: maintain a directed acyclic graph to represent content collaboratively edited by the first electronic device and one or more second electronic devices of multiple collaborating devices, wherein the directed acyclic graph includes a plurality of nodes each representing a respective content object that is created or edited by one or more of the multiple collaborating devices, wherein at least a first node of the plurality of nodes represents a textual content object and at least a second node of the plurality of nodes represents a sketch content object, and wherein each node representing a corresponding sketch content object includes a respective command sequence used to create content of the corresponding sketch content object; during a respective synchronization period, receive one or more editing inputs from one or more devices of the multiple collaborating devices; modify the directed acyclic graph based on relationships between the editing inputs and existing content objects embodied in the directed acyclic graph; determine whether the one or more editing inputs modifies an existing sketch content object represented in the directed acyclic graph; and in accordance with a determination that a first editing input of the one or more editing inputs modifies a first existing sketch content object represented by a first command sequence in a first respective node in the directed acyclic graph, update the first command sequence in the first respective node by merging each individual drawing command included in the first editing input with the first command sequence to produce, in the first respective node, an updated first command sequence of two or more commands that represents the first existing sketch content object as modified by the first editing input. 23. The non-transitory computer readable storage medium of claim 22 , wherein the one or more programs further include instructions, which when executed by the first electronic device with a display, cause the first electronic device to: render the first existing sketch content object represented in the modified directed acyclic graph, the rendering comprising: determining a global position or state of the first existing sketch content object in the content in accordance with a topological traversal of the directed acyclic graph; determining an internal appearance of the first existing sketch content object in accordance with the updated first command sequence in the first respective node; and rendering the first existing sketch content in accordance with the determined global position or state of the first existing sketch content object and in accordance with the determined internal appearance of the first existing sketch content object.