Three short lines, and a whole odd little situation. Someone is sitting in the shade, but there is no sun to cast it. The poem's puzzle is right there: shade is supposed to be a relief from brightness, so what does it mean to sit in it when nothing is shining?
The most natural reading is that the speaker has settled into a habit or a mood. Maybe they're so used to hiding from the light that they keep doing it even when the reason is gone. That could suggest a kind of low, lingering gloom, or a reluctance to step out and enjoy what's available. It could also be gentler: a quiet contentment with stillness, where the speaker just likes being in a dim, calm place.
The poem doesn't tell us which. The line break after "even" slows us down and adds a small pause of surprise. The mood is quiet, a little wistful, and open-ended, leaving readers to decide whether this is sadness, habit, or simple peace.