You are looking at an issue, a bug, in gedit.
Steps to reproduce:
- Open a somewhat larger text file in gedit in a window that is not maximized
- Move the cursor to the last line
- Maximize the window
Expected behaviour: the window is maximized and one sees the current line (the last line) at the bottom.
Observed behavior: the last displayed line is a few lines above. One does not see the line with the cursor.
When you use the left/right arrow keys, the line stays out of sight, which indeed is confusing. However, as soon as you press a left/right arrow key, or start typing, the line pops into view.
If this issue is not known to the gedit developers, it may be worth reporting it, although allegedly, it is not a severe issue.
I cannot reproduce, however, where you appear to suggest that you never can reach the last line. There is a bug reported at Debian in 2016 where the user cannot see the very last line without adding a return to the last line, but I cannot reproduce that in current gedit 3.28.1 in Ubuntu 18.04.
Update 2022-04-26 The bug is still there in Ubuntu 22.04, Gedit 41. The bug report in the mean time has been forwarded to https://gitlab.gnome.org/GNOME/gedit/-/issues/42 without resolution. Is that why Gnome 42 moved to a different standard editor?