Each of the three mouse buttons can be used to manage menus, buttons scrollbars and selection.
Changes cursor position within a page. This means that the new position will be the pointer position, with two exceptions: a click beyond the end of a line (resp. of the buffer) moves the cursor to the end of that line (resp. of the buffer).
If you grab the pointer when the left button is pressed, the standard X selection mechanism runs until the button is released.
A new click releases the selection.
Selects region. The cursor moves at the selected point.
A region is selected and highlighted from the previous location. If a selection already exists, it is extended to the current position.
In standard text window, restores the last selection at the current position. The selected text is inserted before the current position.
In the browser visit window, colors the current buffer.
If a portion of text has been selected, this action erases it. The pointer position is unimportant for this sequence.
When a text region has been erased it can be restored at any position by a MiddleButton click.