mode-line-default-help-echo

mode-line-default-help-echo is a variable defined in `bindings.el'.
Its value is

"mouse-1: Select (drag to resize)\nmouse-2: Make current window occupy the whole frame\nmouse-3: Remove current window from display"

Documentation:
Default help text for the mode line.
If the value is a string, it specifies the tooltip or echo area
message to display when the mouse is moved over the mode line.
If the text at the mouse position has a `help-echo' text
property, that overrides this variable.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.