Shift-click to extend selection does not work

Holding shift key and click elsewhere in the text editor no longer select the text from the current cursor position to where the click is. For example:

1. Open any source/text file.
2. Place the text cursor at the beginning of a line.
3. Holding down the shift key and click the mouse at the end of the same line.

It used to select the text between where the text cursor is and where the mouse clicks. Now it does nothing.