Chapter 3 of the book The Gibbs phenomenon in Fourier analysis, splines, and wavelet approximations by Abdul J. Jerri seems to cover the case of a general orthogonal expansion along eigenfunctions of a Sturm-Liouville problem.
Chapter 3 of the book The Gibbs phenomenon in Fourier analysis, splines, and wavelet approximations by Abdul J. Jerri seems to cover the case of a general orthogonal expansion along eigenfunctions of a Sturm-Liouville problem.