FULL STEPS:

1. Open a moodle course
2. Go to Participants
3. Enrol at least one participant
4. Hover over the "-" button  right Firstname / Surname column title in the main users table

EXPECTED:

The tooltip should say "Hide Full name"

ACTUAL RESULT:

The tooltip says "Hide" without specifying which column

If hovered in the bottom-right corner of the "-" sign, the correct tooltip is displayed. 

BUG:

The problem seems to be that both the <a> and the <i> inside have a title attribute. It is applicable to all tables produced with lib/tablelib.php. The problematic function is flexible_table::show_hide_link in lib/tablelib.php:

...

protected function show_hide_link($column, $index) {
...
return html_writer::link($this->baseurl->out(false, array($this->request[TABLE_VAR_SHOW] => $column)),
{{ $OUTPUT->pix_icon('t/switch_plus', get_string('show')), $linkattributes);}}
...
return html_writer::link($this->baseurl->out(false, array($this->request[TABLE_VAR_HIDE] => $column)),
{{ $OUTPUT->pix_icon('t/switch_minus', get_string('hide')), $linkattributes);}}
...

$linkattributes already contains the correct title, so all that needs to be done is to replace get_string('show') and get_string('hide') with null, so that the icon doesn't have a title attribute.