Removing a list item from a selection using 'ctrl' key is applied on mouse down. Standard selection behaviour should remove a list item on mouse up. You can check this behaviour in windows explorer.
In an application where you use 'ctrl' + drag-and-drop to copy items from a list (like copying files in windows explorer), if you remove item from selection on mouse down, users have to press 'ctrl' after starting drag-and-drop. If you remove item from selection on mouse up, users can press 'ctrl' before or after starting drag-and-drop.
Removing a list item from a selection using 'ctrl' key is applied on mouse down. Standard selection behaviour should remove a list item on mouse up. You can check this behaviour in windows explorer.
In an application where you use 'ctrl' + drag-and-drop to copy items from a list (like copying files in windows explorer), if you remove item from selection on mouse down, users have to press 'ctrl' after starting drag-and-drop. If you remove item from selection on mouse up, users can press 'ctrl' before or after starting drag-and-drop.