Fix transition of middle header in low width screen

basetheme_bootstrap/static/css/basetheme_bootstrap4_pasteur.css

@@ -280,17 +280,17 @@ header .header__middle .dropdown-menu {
 .header__middle.navbar{
     align-items:initial;
 }
-#navbar__middle__collapse:not(.show){
+#navbar__middle__collapse:not(.show):not(.collapsing){
     align-items:initial;
     flex-direction:column;
 }
-#navbar__middle__collapse:not(.show)>div{
+#navbar__middle__collapse:not(.show):not(.collapsing)>div{
     display:flex;
 }
-#navbar__middle__collapse:not(.show)>div:first-child{
+#navbar__middle__collapse:not(.show):not(.collapsing)>div:first-child{
     flex-grow:3.5;
 }
-header:not(.scrolled) #navbar__middle__collapse:not(.show)>div:last-child{
+header:not(.scrolled) #navbar__middle__collapse:not(.show):not(.collapsing)>div:last-child{
     flex-grow:1;
 }