Note that I ask about the longest possible tree and not the tallest possible tree.
There have been a number of questions about the tallest possible tree, and I wrote a detailed answer to one: https://worldbuilding.stackexchange.com/questions/191008/would-a-small-low-gravity-moon-be-able-to-harbor-complex-life/191115#191115
Where I used to live there was a steep slope, somewhat less than 45 degrees I think. And a small tree on that slope fell over in the downhill direction, so that its angle was only a little bit above vertical. Some of the tree roots remained buried in the soil and the tree continued to have green leves for years or decades.
So I wondered about a hypothetical species of trees, perhaps on an alien planet, which grows on slopes and has almost horizontal trunks, casting shade on trees below it and sticking out far from the slope to avoid the shadows cast by the trees higher up the slope.
What factors would limit the growth of those trees to their maximum length, and what would that maximum length be?