I'm afraid I can't answer Maple 18, but I can tell you @Jean-Baptiste R , this is an option I would like them to provide / fix in 2020.1
If one is in the text region, say the header, a right-click (alt-click) does bring up the menu to:
* execute the selection, which it will not perform
* execute the entire worksheet: which it WILL perform
However, if one places the cursor with an execution group, the menu shows no options to execute the worksheet. (But one can hit the execute icon, "!" in the menu bar.) An option in the alt-click menu would be useful.
If one selects a section, or an execution group, there is no option in the alt-click menu to execute either the section or the worksheet. However, the menu bar "!" icon does work. Again, an option here would be useful.
At a minumum, if one is in the text group, or header of a section, and the alt-click menu provides the option to execute the section, it should do it just like it does for the entire worksheet. (Or remove the option unless the section or set of execution groups are selected.
Now that you have pointed out this ability to execute solely an section, I can see how it would be very useful - particularly in presentations.