Ahhhh, line wrapping!!!!!
Try with `Edit->Preferences->Editor->Display->Stop scrolling at last line` unset.
What is happening when you make the editor pane wider with wrapping on is the text on screen occupies less vertical space, so with that setting _on_ it has to scroll (the line at the _top_ of the screen is scroll position) to remove the blank lines past the end of the text.
Then when you narrow the pane again the scroll position is kept at the top of the screen, forcing the current line off the screen as the wrapped text occupies more vertical space again.
I'd say this is intended behaviour, otherwise the user couldn't scroll the display away from the current line and have it stay there.