Why does "cat >> file" not work in zsh with pasted text?

Question

I had never heard of zsh before, but when I updated to Catalina, MacOS pushed me to drop bash in favor of zsh. But I recently have observed some annoying incompatibilities when trying to append to the end of a file using cat >>. The line I want to add is copy & pasted from elsewhere in the terminal:

$ cat >> test.txt
This is a test.

zsh: do you wish to see all 3734 possibilities (1867 lines)? 

In this case I pasted the whole line "This is a test." (including to the end of the line). Unlike bash, zsh will (sometimes?) print the pasted text in reverse video until I press return. In this case I pressed "Control-D" to indicate that I was done pasting in text. At this point, bash would give me the next prompt, but here, zsh gives me a message I don't understand. I'm strongly considering going back to bash. But I'm very curious: what does zsh think it is doing? Does anyone else see this as a bug, or at least a misfeature? It also doesn't seem to happen all the time. Sometimes instead I get the less confusing but still incorrect:

zsh: command not found: This

Answer 1

If you first paste

cat >> test.txt

and press Enter, and then paste

This is a test.

and then

• press Enter followed by ControlD, or
• press ControlD twice,

you will get the result you're looking for (in the first case, with a line break after your input, and in the second case, without).

---

If you directly paste

cat >> test.txt
This is a test.

then the second line won't go to cat as input, but it will go to Zsh as a second line in your command line.

• If you then press Enter, Zsh will try to execute both lines accordingly, leading to zsh: command not found: This.
• If you then press ControlD, Zsh will attempt to list completions for you on the second line, potentially (but not always) leading to zsh: do you wish to see all 3734 possibilities (1867 lines)? (depending on when and where you do this).