After having missed the "perfect horizontal mouse movement" in hierarchical dropdown menus in LibreOffice Writer (Version: 4.4.7.2
Build ID: 4.4.7.2-1.fc22) a few times, and after having noticed that the menu behaviour of Gimp is different and very much more agreeable, I have been wondering about whether the Gimp behaviour could be ported. AFAIK LibreOffice comes with its own menu implementation (from VCL) and Gimp uses GTK+.

Let me illustrate:

Suppose you want to set the "Language" of "All Text" to "None" in LibreOffice. This means navigating *➤ Tools ▼ Language ▶ For All Text ▶ None (Do not check spelling)*. 

It turns out that this is a significant path to trace with the mouse

(See image "Significant Path: Language For All Text None.png")

The menu system of LibreOffice is fickle in that, if you stray from the horizontal line leading into the hierarchically lower menu, the hierarchically lower menu will CLOSE AT ONCE as the next option on the current menu is selected. In the illustration below, you cannot move the mouse along the red path because that means selecting "Word Count". Descending hierarchical menus becomes frustrating as you have to stay very precise (which may be difficult if you are in an uncomfortable position with your hand or are afflicted by neurological disorders).

(See image "Significant Path: Keep the Mouse on the Green Line.png")

On the exact same system, the menu system of Gimp performs very well. Indeed, it seems to have a "target predictor" built in: If you target an element of a hierarchically lower menu, the currently selected option REMAINS SELECTED and the hierarchically lower menu REMAINS OPEN for maximally two seconds as you move the mouse. Both blue and green paths become feasible:

(See image: "Scaling Sartre: Both Paths work.png")

On the other hand, if you target the next item in the current menu, the predictor seems to guess that you want to move to the next option. Thus in the following case, the red path means opening "Colour Tools":

(See image: "Scaling Sartre: Red Path.png")