diff --git a/web/modules/custom/materio_admin/css/term-reference-tree.css b/web/modules/custom/materio_admin/css/term-reference-tree.css
index 3cfd5202..e5bea32d 100644
--- a/web/modules/custom/materio_admin/css/term-reference-tree.css
+++ b/web/modules/custom/materio_admin/css/term-reference-tree.css
@@ -1,17 +1,26 @@
 /**
  * Term reference tree widget: selection tree on the left, sortable track
  * list on the right. Transposed from the retired matminimal theme.
+ *
+ * Flexbox row instead of inline-block columns so the two blocks never wrap
+ * onto separate lines, whatever their content width (the previous
+ * inline-block version wrapped on environments with a narrower form region).
  */
-.term-reference-tree > ul,
-.term-reference-tree aside.term-reference-tree-track-list {
-  display: inline-block;
-  vertical-align: top;
+.term-reference-tree.term-reference-tree-track-list-shown {
+  display: flex;
+  flex-direction: row;
+  align-items: flex-start;
 }
-.term-reference-tree > ul {
-  padding-right: 1em;
+
+.term-reference-tree.term-reference-tree-track-list-shown > ul {
+  flex: 1 1 45%;
   min-width: 45%;
+  padding-right: 1em;
+  margin: 0;
 }
-.term-reference-tree aside.term-reference-tree-track-list {
+
+.term-reference-tree.term-reference-tree-track-list-shown aside.term-reference-tree-track-list {
+  flex: 1 1 55%;
   border-left: 1px solid #aaa;
   padding-left: 1em;
 }