No, such a feature is not available in Textastic.

On iPad, there are undo and redo buttons in the top toolbar above the editor when you are editing a file. Undo is an arrow pointing to the left, redo is an arrow pointing to the right.

On an external keyboard, you can use Cmd-Z to undo and Cmd-Shift-Z to redo.

Please make sure the files referenced by the index.html file are referenced using relative urls (e.g. "styles/main.css" and not "/styles/main.css").

Then the preview should work as expected. 

Here's a tip on how to select long sections of text using the on-screen keyboard (without navigation wheel and without an external keyboard):

• Tap in the editor to place the insertion point at the start of the selection
• Hold shift on the on-screen keyboard
• Tap in the editor where you want the selection to end. This will make a selection from the insertion point set above to the location you tap at. (Make sure to not tap on a line number when doing that.)

Ok, this is how the cursor navigation wheel is supposed to work:

You can use it to change the text selection using the buttons on the wheel (exclusively) instead of the selection handles.

There are two buttons in the middle:

1. If none of the two buttons is active: this means that there is only a text insertion point and no text selection. You can use the arrow keys on the cursor navigation wheel to move the text insertion point.
   
   If you tap the top button, the word to the left of the text insertion point is selected. You can now move the start of the selection with the arrow keys.
   If you tap the bottom button, the word to the right of the text insertion point is selected. You can now move the end of the selection with the arrow keys.
2. The top button is active: this means that you are moving the start of the selection with the arrow keys on the cursor navigation wheel. Tapping the button again removes the start of the selection and the end of the selection becomes the insertion point. Tapping the bottom button lets you move the end of the selection.
3. The bottom button is active: this means that you are moving the end of the selection with the arrow keys on the cursor navigation wheel. Tapping the button again removes the end of the selection and the start of the selection becomes the insertion point. Tapping the top button lets you move the end of the selection.

The cursor navigation wheel does not currently interact with touches on the code editor itself.

Please note that the cursor navigation wheel was added many, many years ago, when text selection worked very differently and there was no trackpad/mouse support. I have considered removing it a few times, but kept it in for those users who might still be using it.

Tapping or clicking a line number currently always selects that line and doesn't take into account whether Shift is pressed on a keyboard.

I'll look into improving this behavior.

If you shift-click a line number, would you expect that whole line to be included in the selection or not? Sublime Text and Visual Studio Code do include the line in the selection.

Can you please explain the problem in more detail?

Maybe the detailed manual can help: https://www.textasticapp.com/v10/manual/

I've just followed the instructions at 

https://support.google.com/mail/answer/9212652?co=GENIE.Platform%3DiOS&oco=1 and sent feedback about this to Google.