x-stretch-cursor

x-stretch-cursor is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means draw block cursor as wide as the glyph under it.
For example, if a block cursor is over a tab, it will be drawn as
wide as that tab on the display.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.