use a more specific selector

common/gtk-3.0/3.20/gtk-dark.css

@@ -579,7 +579,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #2b2e39; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/gtk-darker.css

@@ -578,7 +578,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #5294E2; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/gtk-solid-dark.css

@@ -579,7 +579,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #2b2e39; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/gtk-solid-darker.css

@@ -578,7 +578,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #5294E2; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/gtk-solid.css

@@ -578,7 +578,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #5294E2; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/gtk.css

@@ -578,7 +578,7 @@ button {
 .linked:not(.vertical):not(.pathbar) > button:checked + entry, .linked:not(.vertical):not(.pathbar) > button:checked + spinbutton {
   border-left-color: #5294E2; }
 
-.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):hover {
+.linked:not(.vertical):not(.pathbar) > entry + button:not(:checked):not(:active):not(:only-child):hover, .linked:not(.vertical):not(.pathbar) > spinbutton + button:not(:checked):not(:active):not(:only-child):hover {
   box-shadow: none; }
 
 .linked:not(.vertical).path-bar > button:hover:not(:checked):not(:active):not(:only-child):hover {

common/gtk-3.0/3.20/sass/_common.scss

@@ -578,7 +578,7 @@ toolbar.inline-toolbar toolbutton {
   > button:first-child:hover:not(:checked):not(:active) { box-shadow: none; }
   > button:checked + button:not(:checked):not(:active):hover { box-shadow: none; }
   > button:checked + entry { border-left-color: if($variant=='light', $selected_bg_color, $entry_border); }
-  > entry + button:not(:checked):not(:active):hover { box-shadow: none; }
+  > entry + button:not(:checked):not(:active):not(:only-child):hover { box-shadow: none; }
 }
 
 // special case for path-bars