In LilyPond, why is the first (yellow marked) measure not aligned with the other measures? What is the way to fix it?
In LilyPond, to get rid of indentation at the start of the piece of music you just need to include
indent = 0 in the layout block.
The reason the indentation is there, is to leave some space for the instrument name.
It also makes the first line of the piece stand out (somewhat like indenting paragraphs in a body of text). This is especially useful when there are multiple piece over several pages.