[TxMt] Viewport position lost when switching tabs with larger font sizes

[James Mead]

Steps to reproduce:

* Set font size to larger than the default.
* Position the cursor towards the bottom of a file such that the
viewport is displaying the bottom part of that file.
* Switch to another tab.
* Switch back to the original tab.
* See that although the cursor is still in the same place, the
viewport is now displaying an earlier part of the file.

It looks as if the viewport position calculation is not taking into
account the larger font size.

Cheers, James.