Hi,
after updating my OSX to _macOS High Sierra (version 10.13.2)_ some lines of my .tex files keep disappearing. As you can see in this picture some lines don't appear in texstudio but they are shown in texmaker. I've used the same version of texstudio and the same files before updating my mac and I didn't have such problems.
_TeXstudio 2.12.6 (hg 6631:16db084dae24)
Using Qt Version 5.7.1, compiled with Qt 5.7.1 R_

I can't test on OSX 10.13.
Could you try a development build: https://bintray.com/sunderme/texstudio/download_file?file_path=texstudio-osx-2.12.7-201712272142-git_1e7fa04.dmg
@sunderme I tried this and the same problem exists. :(
Can you please activate the line numbers (Options -> Adv. Editor -> Appearance) and redo the screenshot. Some lines appear to be cut of in height and with the line numbers it's easier to see.
@timhoffm Here is a screenshot with line numbers activated. I have marked some of the disappeared lines with bookmarks.

Interesting. The font appears to be much larger than the line height. Does it help to manually increase Options -> Editor -> Line Spacing?
@timhoffm Yay!! That's it! Thanks a lot! I really missed using TeXstudio!
The below picture is the result of increasing line spacing to 180%

It seems that changing font size and font family don't have any effect on the lines containing Farsi characters!
Here I have changed the font family to comic sans and increased the font size but Farsi parts haven't changed at all!

It seems that changing font size and font family don't have any effect on the lines containing Farsi characters!
At least that explains why the farsi texts were larger than the line height. I don't know how the font substitution works. It's either done by Qt or by the OS. Given that this broke after updating the OS, I suppose that there is a bug in OSX 10.13.2 concerning font substitution.
@01000110 could you provide a text snippet for testing?
@jonasstein This is a text snippet for you :)
example.tex.zip