This functionality was removed in:

http://git.gnome.org/browse/gnome-shell/commit/?id=907fc2f067c8c831ca16ef5722026ff481b8cb72

Though appIcon.js where the behavior was implemented was accidentally not removed and left around until:

http://git.gnome.org/browse/gnome-shell/commit/?id=8c178378aec921d1b3437bc0f2139f111c8624fa

Probably what happened was something like Colin implemented things in a straightforward way first and forgot to go back and pick up the special convoluted corners later.

Created attachment 155746 [details] [review]
[AppDisplay] Popup the menu on left-click/hold

Left-clicking an app icon and holding the button used to pop up the
app menu, but regressed when rewriting appDisplay.
Restore the previous behavior.

Cool.  Seems to work again.

Review of attachment 155746 [details] [review]:

Looks good to commit except for one style thing.

::: js/ui/appDisplay.js
@@ +469,3 @@
+                if (this._menuTimeoutId != 0) {
+                    Mainloop.source_remove(this._menuTimeoutId);
+                    this._menuTimeoutId = 0;

I think I'd bundle up these lines into a _removeMenuTimeout() function. Since you have it in 5 places or so.

Attachment 155746 [details] pushed as d9f43e2 - [AppDisplay] Popup the menu on left-click/hold