Clough's Theorem (a variation of Viviani) and some Generalizations

During 2003, Duncan Clough, a Grade 11 student from Bishops Diocesan College, a high school in Cape Town, was exploring Viviani's Theorem, which says that the sum of distances of a point to the sides of an equilateral triangle is constant. Using dynamic geometry software, he then discovered (but could not himself prove) the following interesting variation of Viviani's theorem.

Clough's Theorem: Label the feet of the altitudes from an arbitrary point P inside an equilateral triangle ABC to the sides AB, BC, AC respectively as PC, PA, PB, then APC + BPA + CPB is constant. (Drag point P in the sketch, or B or C to change the size or orientation of the triangle.)

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Can you logically explain (prove) why the result is true? Can you find different explanations (proofs)?

Exploring Some Generalizations
Viviani's Theorem generalizes to polygons that are equilateral or equi-angled, or to 2n-gons with opposite sides parallel - see for example, 2D Generalizations of Viviani's Theorem. Can you similarly generalize Clough's Theorem to these higher polygons? See Further generalizations of Clough's Theorem.

Some Proofs
1) Read my ICME 12 Regular Lecture in Korea in July 2012, published in Pythagoras, An illustration of the explanatory and discovery functions of proof for various proofs of the above.
2) Read the 2012 proof of Clough's Theorem by Shailesh Shirali from India in the mathematics education journal "At Right Angles" at: Viviani's Theorem and a Cousin.
3) Clough's Theorem was used as a problem in the first round of the British Mathematical Olympiad of of 2013/2014. Read one of the proofs produced by students, which was similar to the following very short and elegant one, kindly sent to me in 2015 by Gregoire Nicollier from Switzerland at: Clough's Theorem: The Simplest Proof.