horizontalScroll method
Scrolls the day-axis by one wheel notch — the Shift+wheel horizontal scroll
(#65). forward (a wheel-down notch) reveals later columns by decreasing
x; the reverse reveals earlier ones. The step is the flat
PlannerConfig.scrollStep: unlike the time axis, columns are a fixed
PlannerConfig.blockWidth wide and don't scale with zoom, so a notch always
moves the same distance. Clamped to the same [_maxXOffset, _minXOffset]
bounds as a horizontal drag, so it can't reveal past the grid edges.
Implementation
void horizontalScroll(bool forward) {
final step = config.scrollStep;
x += forward ? -step : step;
if (x > _minXOffset) {
x = _minXOffset;
} else if (x < _maxXOffset) {
x = _maxXOffset;
}
}