It's very hit and miss in terms of code editors. For example Sublime Text has no complex text shaping at all. It actually renders Arabic as disjointed letters.
The JetBrains series of IDEs have some Arabic support, in the sense that the text is rendered properly at least.
I made an Arabic Markdown editor for the Mac last year, so that personally helps me when I need to edit Arabic text http://katibapp.com/
There's a lot left to be done for Arabic. So much low-hanging fruit.
In particular, can Emacs deal with cases where the Unicode RTL algorithm gives nonsense, such as editing HTML in Arabic?
(Because HTML tags are made of punctuation, Roman letters, and attribute values that could be in any language, you can end up with LTR segments that may include half a tag and some adjacent text. At that point you can't even see visually what they include; you have to reason out what Unicode is doing. You might even end up with a tag that begins with an open angle bracket that looks like > and ends with a closing angle bracket that also looks like >.)
I ask this not as someone who knows any Arabic, but someone who has to deal with the results of complexities of text such as RTL formatting. To make mixed-direction text editing "perfect", someone would have to put a lot of task-specific design into it.
!ممتاز
I am always disappointed when I try and type something in Arabic on OS X. Now I will install Kawkab-mono and Katib both, and improve matters 200%.
The JetBrains series of IDEs have some Arabic support, in the sense that the text is rendered properly at least.
I made an Arabic Markdown editor for the Mac last year, so that personally helps me when I need to edit Arabic text http://katibapp.com/
There's a lot left to be done for Arabic. So much low-hanging fruit.