Web editor shows N+1 lines in a file with N lines for N>0.

Web editor shows N+1 lines in a file with N lines.

What I see in Vim editor:

1 This is the first line.
2 This is the second line.

What I see in the web editor:

1 This is the first line.
2 This is the second line.
3

No editor I know of shows N+1 lines unless N=0 thus this is an unexpected behavior that tells me there is a spurious line ending on line 2 (Vim tells me it is not).