Fix widget-reusing code

Issue #1187
Pass doc to lineIsHidden

There's no guarantee that we'll be able to get the document from the parent
link (might not be set).

Closes #1199
Make coordsChar take page coordinates again

It had accidentally started taking window coordinates in the 3.0 refactor.
[htmlmixed mode] Remove obsolete property
Only prevent re-focusing of textarea on mobile platform

Re-focusing is harmless on desktop browsers, and helps paper over
some browser bugs (such as firefox not 'really' focusing a textarea
in an iframe when the textarea is focused before the frame gains focus)