画布扩展支持配置按实际大小扩展，而不是每次扩展一块初始画布的大小

### 功能描述

画布扩展支持配置按实际大小扩展，而不是每次扩展一块初始画布的大小。像下面这样，每次扩展一整块画布，会有太多的留白处


### 期望解决方案

画布扩展支持配置按实际大小扩展

The issue provides only a description and GIF, with no file, test, or entry point named. Start by locating the canvas-expansion implementation and its existing tests, then determine how expansion currently uses the initial canvas size. Done means configurable expansion can use the actual required size without unnecessary blank space.

Written by the indexing model from the issue text.