Expand form container to fill all parent spaces

Closes #13073
(grafted from 882083de49d111b61d7e525e0fb0f8f43fa13835)

--HG--
branch : 7.0

sao/src/sao.less

@@ -633,6 +633,8 @@ img.icon {
     width: 100%;
     .form-container, .form-hcontainer, .form-vcontainer {
         display: grid;
+        width: 100%;
+        height: 100%;
     }
     .form-item {
         display: flex;