Confirmed.
For now please use below code for workaround. We will fix this in the one of the next builds.
procedure TMainForm.UniFormReady(Sender: TObject);
begin
with cArea, JSInterface do
begin
JSCode('var b = document.getElementById("'+JSName+'_canvas"); b.removeEventListener("mouseup", '+JSName+'.ev_mouseup, false);');
end;
end;