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When multiple menu items point to the same page, the active trail is selected from the higher-preference menu, even if the item in that menu is disabled.
Consider two menus:
Navigation - Nav Page One (node/1) -- Nav Page Two (node/2) [disabled] MyMenu - My Page One (node/1) -- My Page Two (node/2)
The active trail will show the Navigation menu item (Nav Page One) even though the menu item for node/2 is disabled.