Mirror DatumCanvas/Konva: hit-test on every mousedown/touchstart, and
let an existing-measurement hit win over both stage pan and placement
tools. Empty-space presses fall through — to a placement (if a tool is
active) or to a pan.
Suppress the trailing click event when the press picked up a prior
measurement so the placement tool doesn't commit a spurious new point
right after a corner drag ends.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bb8ea1929f