How to differentiate between selecting a whole line via margin click or mouse drag

Hello,

You would need to look at the selection's range via properties on editor.ActiveView.Selection.  A line selection via clicking on the line number will be the start of the previous line through the start of the current line.  A mouse selection of a line would likely be from the start of the current line through to the end of the current line.

Thanks for your reply. However when we select the last line, the line selection will only select the start to end of current line. So for the last line is there currently no way to differentiate between line and mouse selection?

That is correct since there is no offset for the caret to move to on the next line, as that would be beyond the end of the document.