The menu entry like "Zoom" in the image http://www.kensan.it/tmp/menu.png can be a long row like the entry in the menu bookmarks in the firefox menu or other menus. If you must choice an item like "zoom in" (see png) you must to walk the entire row with the mouse pointer. I the row is long the mouse point walking is not straightness so are open the upper menu or the lower menu (sidebar menu or page style menu, see png).

To avoid this the feature that I proposed is to add an optional click in the menu "Zoom" that block/fix the entry "Zoom". So if you travel with the mouse pointer outside the row "Zoom" the entry Zoom is always selected.

For Deselect the entry menu block you can click other menu entry.

Reproducible: Always

Steps to Reproduce:
1.click the menu View (see http://www.kensan.it/tmp/menu.png)
2.descend to the entry "zoom"
3.click (one click) on the entry "zoom" (whatever point of the row "zoom")
4.travel from the begin of the row "zoom" to the right end because you want to choice the voice "zoom in" (see png)
5.go outside the row "zoom" because the row is long and your mouse pointer path is not straightness

Actual Results:  
The menu entry "Zoom" is deselected and the sub menu disappear

Expected Results:  
The menu entry "zoom" remain selected and you can selected the voice "zoom in".