Find the solution myself:
It was a style (theme) problem. Checked again using the default theme and this behave as expected. So I inspected carefully the other themes and found that not all those themes had this problem.
The solution was to edit the theme's code (twilight.xml) to insert the following lines:
<style name="selection" background="orange" />
<style name="selection-unfocused" foreground="white" background="gray" />
By the way, the location of the themes in gEdit 3 differ from gEdit 2. Here it is:
~/.local/share/gtksourceview-3.0/styles/
You can create the directory yourself if it doesn't exist. Further info on style paths here:
http://live.gnome.org/GtkSourceView/StyleSchemes
You are looking at an issue, a bug, in gedit.
Steps to reproduce:
1) Open a somewhat larger text file in gedit in a window that is not maximized
2) Move the cursor to the last line
3) 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.
Best Answer
You can remove the tab space from multiple lines by selecting those lines and pressing Shift+tab. Shift+tab is used to give a reverse tab, i.e. tab in the opposite direction, so this should work almost wherever you want to reverse tab.