Blockly's context menu adds undesirable scrolling near bottom of the window

If you right-click on the Blockly workspace near the bottom of the window, the context menu extends beyond the bottom, and scrolls the whole window up a bit. It's jarring and looks broken. Not a big deal, though.

首先，在 Blockly 工作区中打开窗口底部附近的上下文菜单，以重现该问题。跟踪上下文菜单的定位和页面滚动行为；当无需滚动整个窗口即可使用该菜单时，即表示完成。

由索引模型根据 Issue 内容生成。