Editor: cursor stays put in tables, slash popup anchors to the caret

Two small editor fixes that you will feel every time you write:

• Cursor in tables. When you were editing inside a table cell, the cursor would occasionally jump out of the cell after a re-render — usually when something else on the page caused a redraw. The cursor now stays exactly where you left it.
• Slash command popup. The slash menu used to anchor to the line you were on rather than the precise caret position, which meant the popup floated next to the start of the line instead of next to the actual / you typed. It now anchors to the caret, where you expect it.

Tiny fixes, big quality-of-life win once you notice them.