Large files do not render correctly

[albe-jj]

If I open a file with more 1000 lines it does not not display correctly after a certain point it only shows blank lines.
Also for these large files widget uses a lot of CPU and RAM.

[peter-kal]

I saw that the problem of lines is fixed by adding height to the GutterStyle property. The performance is good for me, but the situation worsens when typing.

[peter-kal]

It seems already discussed in other issues, like #255.