Spatium rounding problems due to file format and UI
This issue ties into this other issue: https://musescore.org/en/node/278887
Now I'm remembering all the reasons I spent so much effort on my previous Pixels as Page Size Units PR over a year ago. The page settings dialog is doing some unnecessary and erroneous work with page size units. The result is rounding errors in the score's spatium value.
Simplest reproduction steps are to create a new score, switch the page size units from mm to inch and voila! your score spatium is no longer 25, it's 24.8400000000000003. A rather large rounding error that causes various downstream processes to do extra scaling work unnecessarily. I have reported on this rounding issue in the past, around 2 years ago. I thought it was resolved, but now I'm remembering that maybe the solution was buried in my rejected Pixels as Page Size Units PR.
This is a more serious issue than it might seem. Rounding errors like this compound themselves with each iteration. Incorrect and unnecessary scaling impacts even the core layout procedures that have undergone massive transformation in 3.0. It definitely affects SVG export in a negative way, by forcing full matrix transformations on the entire score, on top of the incorrect and unnecessary scaling.
I'm marking this as critical, though that might be an overreach. There needs to be a discussion about page settings and page size units that includes this significant error and the overall UI paradigm raised in the other issue linked at the top of this posting. There is a holistic solution to all of these problems, and I am willing to code it because of the impact to my project and the fact that I have already dived deep into this, albeit over a year ago.
edit - Here is the original issue I filed in April 2016, with comments as recent as 6 months ago:
We can mark this as duplicate and revive that issue if you prefer.