Abandonment
Yes. Abandoning a game in progress shows the same poor sportsmanship online as it would if you did it over the board. If you are going to take the loss, resign and move on to the next game.


https://support.chess.com/en/articles/8562517-how-do-i-report-someone
Report players if you feel they arr not fair
Abandoning game is just childish behave and such person has not any kind of good sportsmanship. If you are losing a game you either resign or then play to bitter end. Even not resigning even in absolutely lost position is better than just abandoning game either by intentionally cutting connection or letting time running out. Such behave is just really stupid because you would lost anyway and you are seen in bad light.

resing shows good sportsmanship, but I have to admit that I often just close the tab when it gets annoying

It is horrible sportsmanship and quite annoying, but thankfully as you get higher in elo less people do this. Just more motivation to keep working hard to improve!