In order to get the bug SHOWN IN THE SCREENSHOTS to happen first you have to type some short text in the chat and press enter then type something long enough that the text goes a bit to the left and press enter again. Next you have to open the chat again an press the "up" arrow twice and you'll see that the short text is a bit to the left like the very long one.
MC-95609
Chat Bug Thingy
Resolved
Duplicate
0
I see, if you do
and then do
and then press up arrow once, it goes to the end of the '/say' command (so the left side starting with /say has scrolled off the screen), which is expected behavior...
...and then if you further press up arrow again, the short /kill command is also 'scrolled to the left', despite that fact that it would fit fine on the screen, and this is the unexpected bit.
(I agree this is a bug, though a minor one.)