This is based on work by Radek Piórkowski <radek25c@gmail.com>.

Closes #4587

Issue #4575

Issue #4575

Closes #4589

Closes #4569

Where it would return a nonsense offset when called for the end of
the document if the last line had line widgets above it.

Issue #4557

Since it's increased the strangeness of the userAgent string again

Issue #4408

Since this raises an exception in IE8