With two or more objects selected and no key object, a click on Align
to Key Object in the Align panel stored Key as the Align To choice and
set no key. The align buttons of the Align panel, the Control bar and
the Properties panel then ran object.align with to: "key", which fails
with "no key object". Distribute Spacing sent the panel's spacing
value and kept the leftmost or top object in place. The stored Key
stayed until Align to Selection, Align to Artboard or Cancel Key
Object was chosen.
The button now runs select.key on the topmost selected object, the
last in paint order, whatever order the objects were selected in, and
the canvas draws that object with the key-object outline. The align
buttons in all three places and Distribute Spacing then use that key,
and Cancel Key Object clears it. A key that is already set stays the
key. With fewer than two objects selected the button is disabled, as
the Distribute buttons are. The button stores no choice, so once the
key is cleared by Cancel Key Object, by a click on the key object or
by another selection, Align To shows the stored Selection or Artboard
choice again. Cancel Key Object no longer sets that choice to
Selection.
Docs: docs/target-app-parity.md.
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