Scenario:

• grab a pane near the left of the viewport
• edit the pane by pressing C-e

Now key handling breaks in lots of ways. When the cursor is invisible in the grabbed pane you can't edit even when it's visible on the surface. Down arrow moves to strange areas.

The cause: we're trying to edit the pane on the surface, but cursor_x and cursor_y were being clobbered when rendering Grab_pane.