A Git color setting produced a terminal sequence that diff-so-fancy did not fully recognize. Written with a visible ESC in place of the escape byte, it looked like:
ESC[1;38;2;133;153;0;48;2;0;43;54mThat is eleven numeric fields: one for bold, five for the truecolor foreground, and five for the truecolor background. The existing expression allowed only seven.
My patch changed the repetition after the first field from {0,6} to {0,10}. It also added a fixture with actual truecolor sequences and checked that the leading diff + and - markers were removed as expected.
The symptom was a line marker left behind. The cause was earlier in the line: a valid color prefix exceeded the bound the parser used before processing the marker. Treating the color as decoration did not remove its effect on the parser’s position.
The change merged the next day. It enlarged one grammar bound to accommodate the specific combined attributes Git was emitting and supplied the corresponding regression case.
That tool also appears in my pager configuration. Using it on ordinary diffs supplied the input shape its smaller color model had missed.