GTK3,4: Minor logic fixes

src/gtk3/widgets/_button.scss

@@ -391,12 +391,12 @@ combobox {
 	}
 
 	> box > button.combo:only-child {
-
 		//Force shadow for single combobox buttons
-		@include neobutton(normal)
+		@include neobutton(normal);
 
 		// remove a shadow for headerbar comboboxes
-		headerbar & {
+		headerbar &,
+		.linked & {
 			box-shadow: none;
 		}
 

src/gtk3/widgets/_entry.scss

@@ -208,7 +208,7 @@ treeview entry {
 	}
 }
 
-combobox > box > button.combo {
+combobox > box > button.combo:not(:only-child) {
 	background: none;
 	@include entry(normal)
 

src/gtk4/widgets/_button.scss

@@ -384,12 +384,12 @@ combobox {
 	}
 
 	> box > button.combo:only-child {
-
 		//Force shadow for single combobox buttons
-		@include neobutton(normal)
+		@include neobutton(normal);
 
 		// remove a shadow for headerbar comboboxes
-		headerbar & {
+		headerbar &,
+		.linked & {
 			box-shadow: none;
 		}
 

src/gtk4/widgets/_entry.scss

@@ -208,7 +208,7 @@ treeview entry {
 	}
 }
 
-combobox > box > button.combo {
+combobox > box > button.combo:not(:only-child) {
 	background: none;
 	@include entry(normal)