`read` clears last line if it doesn't end with a new line

[Yutsuten]

fish, version 4.3.3
Kernel: Linux 6.12.63-1-lts
Terminal: Alacritty

The issue happens even without third-party customizations (sh -c 'env HOME=$(mktemp -d) XDG_CONFIG_HOME= XDG_DATA_DIRS= fish')

---

This issue would not happen before 4.3.

If you use read in a line that contains text, for example after echo -n, read will clear that line's text and put itself at the first column of that line.

For example:

echo -n 'Press ENTER to continue...' && read --prompt-str ''

Before 4.3, it would print Press ENTER to continue... and put the cursor at the end (| bellow symbolizes the cursor):

Press ENTER to continue...|

In 4.3, that line's text is erased by read and output becomes:

|

It happens even without --prompt-str:

echo -n 'Press ENTER to continue...' && read

read> |

(Yes, I know there are better ways to use read, but this is not the expected behavior nevertheless)

[krobelus]

as far as I can tell, echo -n foo; read has been broken for a long time; before 4.3 fish would draw the read> prompt wherever the cursor is, but then carry on with the assumption that cursor is at x=0, making the cursor jump as soon as you type anything but enter. Now we clear the line to make the assumption true.
But you're right that echo -n foo; read --prompt-str '' evidently worked fine, due to the empty prompt str, so we can use the old behavior in that case.

[krobelus]

worked around it for now for this specific case in cfadb2d, but we stil want a proper fix