The attached test cases show that high multipliers in viewport scaling lead to broken text selection. The glyph seletion does not follow the mouse cursor any more in the first of the two inline SVG images.
Created attachment 186409 [details]
Selection inaccuracy with viewbox scaling (500x)
Created attachment 186410 [details]
Selection inaccuracy with viewbox scaling (100x)
Found this while trying to find a test case for bug 108479.
Test cases work for Firefox and Chrome, and fail in Safari.