Raised by plc-user in discussion #618: the diagram editor allows moving
elements by as little as 1px, and asked that rotation not undershoot
that. Checking the actual constraint (Settings -> DiagramEditor_xGrid_sb
/ _yGrid_sb, both independently configurable, minimum 1, maximum 30)
turned up a real, verified gap this PR's existing fractional-pivot fix
doesn't cover: an ASYMMETRIC grid (xGrid != yGrid).
Swapping X/Y deltas for a 90-degree turn -- the exact-arithmetic path
already in this file -- only stays on the configured grid if
xGrid == yGrid. With an asymmetric grid, a delta that was a clean
multiple of xGrid lands on the Y axis after the swap, where the grid
unit is yGrid, and one is not generally a multiple of the other.
Verified on a real build (not just derived): two elements at (100,210)
and (150,420), both on-grid under xGrid=10/yGrid=7, selected and
group-rotated 90 degrees via a temporary local CLI harness.
before this change: (242,292) and (32,342) -- off-grid on both axes
after this change: (240,294) and ( 30,343) -- exactly on-grid
Confirmed the same drift is present without this change too (i.e. not
something introduced elsewhere) and that xGrid==yGrid, the common case,
is unaffected: snapping an already-on-grid point is a no-op.
Fix: re-snap the final computed position to Diagram::snapToGrid(), not
just the shared pivot, for the exact-90-degree path. Left the
arbitrary-angle trig fallback alone -- it has no caller today (the
diagram editor only ever passes multiples of 90) and "on-grid" doesn't
have a clean meaning for an arbitrary angle regardless of grid shape.
Does not attempt to fix a separate, pre-existing property surfaced
while testing this: four consecutive 90-degree turns do not reliably
return a selection to its exact starting position, even on a symmetric
grid, because each RotateSelectionCommand recomputes the pivot fresh
from the selection's current sceneBoundingRect(), and an item whose
bounding box isn't rotationally symmetric reports a different box (and
therefore a different centre) at 0 and 90 degrees. Verified this drift
is identical with and without this change, so it is not a regression --
just a different, harder guarantee this change does not attempt.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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