I think the most correct answer to this question may actually be here in this link, link, searching for words like "edit" "text" or "writer" .
Also, people replying to the question linked to this one claim the following 2 computers would of been capable 1st link and 2nd link, although there's no evidence it was done in terms of a full alphabet and keyboard etc .
Another possibility or answer to this question ( too difficult to find information on ) is that the 'earliest real-time text editor' ( displaying on a crt ) may in no way have been connected to a computer, it may have simply been some sort of simple electromechanical system containing a video-memory somehow using something like Charactrons ( Charactron, Typotron, Monoscope, and the very earliest the Monotron ) with the video memory just storing coordinates linked to a specific character ), all done without a computer, but using a keyboard, or some other similar method but not using things like Charactrons but using some other type of video memory.
Also not forgetting the Motograph patented in 1923 link, the rolling text adverts up on buildings made of light-bulbs, individual letters were placed in frames on a conveyor belt which touched electrical contacts .
For the PDP-1, the first machine was actually delivered in 1959, there seems to have been a crt ( publicised with the tv type crt screen in 1959 ), which displayed text.
I used to have a much higher quality photo of the text on the crt, where you could magnify it and see the font, but now I can only find the image below, and also the image without the text.
The images shown below, are from the following sites -
- information and photos
I have no further information about the text on the PDP-1 crt, although maybe it's related to the above mentioned TVEDIT .
You may already know about the following question, not sure if it's a duplicate, which includes a question and information ( in the comments ) about some radar screens which displayed text, and a machine in the late 1940's - First computer to have interactive text on a CRT?
The Whirlwind computer in 1949
Quote from this website webpage "Whirlwind went operational in 1949. It
was the first digital computer capable of displaying real time text and graphics on a
video terminal, which was an oscilloscope screen."
Searching for keywords like "whirlwind" "crt" "text" "editor"gets results like this link
- the small desktop sized, crt screen calculator, Friden EC-130 of 1963, which was then copied by other companies .
- the IBM 2260 of 1964 ( information and photo , wikipedia page )