A real, reproducible bug reported as 'misplaced sometimes by a few
small squares': Coord.as_fraction() centers a sub-cell at x + 0.5 (so
a marker drawn at its own coord's exact pixel position round-trips
back to the same coord on click), which meant point_to_coord()'s own
rounding was landing exactly on a .5 boundary, the single worst case
for floating point, tiny representation error from the col/row math
upstream could tip round() to either side and silently return a coord
one sub-cell off from the one actually clicked.
Reproduced with zero pixel math involved at all, just feeding
Coord(...).as_fraction() straight back into point_to_coord(), ruling
out the zoom/pan refactor or the legend margin as the cause (both were
suspected first). Fixed by subtracting the 0.5 offset before rounding,
which recovers a value that's supposed to be an exact integer instead
of an exact half-integer, round() is robust to tiny float noise around
a true integer, just not around X.5.
Verified exhaustively (all 20,000 possible coordinates round-trip
correctly now, not just a handful of samples, since the original bug
was itself float-pattern-dependent) and locked in with a permanent
regression test.
Dominik Roth
512a0a41b4