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.

Start by reproducing the issue in the Blockly workspace by opening the context menu near the bottom of the window. Trace the context-menu positioning and page-scroll behavior; done means the menu remains usable without scrolling the whole window.

Written by the indexing model from the issue text.