마우스 드래그 기능 개선

src/renderer/handlers/selection.js

@@ -88,7 +88,8 @@ function _onStart(e) {
     height: '0px',
   });
 
-  if (!e.shiftKey) {
+  const preserve = e.shiftKey || e.ctrlKey || e.metaKey;
+  if (!preserve) {
     _state.selectedIdxs.clear();
     lastSelectedIdx = null;
     _cb.refreshCardSelection();
@@ -113,9 +114,10 @@ function _onMove(e) {
     const r = card.getBoundingClientRect();
     const overlaps = !(r.right < sel.left || r.left > sel.right ||
                        r.bottom < sel.top  || r.top  > sel.bottom);
+    const preserve = e.shiftKey || e.ctrlKey || e.metaKey;
     if (overlaps) {
       _state.selectedIdxs.add(i);
-    } else if (!e.shiftKey) {
+    } else if (!preserve) {
       _state.selectedIdxs.delete(i);
     }
   });