Fix Grid unbuffered editor validation error area theme (#18812)

Change-Id: I0257558f97312a28639773ded12e732fea6f1327

WebContent/VAADIN/themes/base/grid/grid.scss

@@ -425,6 +425,7 @@ $v-grid-details-border-bottom-stripe: 1px solid darken($v-grid-row-background-co
     margin-top: nth($v-grid-cell-horizontal-border, 1) * -1;
     background: $v-grid-row-background-color;
     padding: 0 5px;
+    width: 100%;
 
     + .#{$primaryStyleName}-editor-cells > div {
       border-bottom: none;