You can add custom syntax definitions to Textastic by using TextMate bundles or Sublime Text packages. See
https://www.textasticapp.com/v9/manual/customization/custom_syntax_themes_templates.html

A search for "Z80 Assembly Sublime Text" brought up this Sublime Text package which should work:

https://github.com/mrcook/Z80Assembly

Just put the file "Z80Assembly.tmLanguage.sublime-syntax" into the folder "Local Files/#Textastic/Z80/" and reload customizations. Then you should be able to select "Z80 Assembly" as a syntax definition in the file properties.

Here's how to get the file on an iPad:

• Go to https://github.com/mrcook/Z80Assembly in Safari.
• Tap the green "Code" button and select "Download ZIP".
• Unzip the file in the Files app.
• Copy the file "Z80Assembly.tmLanguage.sublime-syntax" to "On my iPad/Textastic/#Textastic/Z80/".

Thanks, especially for the details on how other editors do it. I'll consider it for a future update.

Thanks for letting me know, I'll fix it in an update.

This already works. Select Textastic -> Customizations -> Show Customization Folder from the main menu and put sublime syntax files in a subfolder, then reload customizations. This should behave the same as the #Textastic folder in Textastic for iPad and iPhone.

You can add custom fonts. This is explained in the manual at https://www.textasticapp.com/v9/manual/customization/custom_fonts.html

Not yet, sorry.

It's great to hear that it seems to work now. Let me know if it happens again. 

Please also try if a reboot of your iPad changes anything. I have seen keyboard problems go away after a restart. 

I tried to reproduce this on my iPad Pro (12.9-inch, 4th gen) connecting to Ubuntu 22.04 LTS, but it worked fine every time. I used landscape orientation, no external keyboard and had additional keys disabled.

Here is a screenshot of how this looks right after connecting:

Before I set up a new Xubuntu 20.04 VM, can you please send me a short screen recording video of the problem? Maybe I can see something.

Thanks, I'll try to reproduce it.