fix briefly disappearing data layer during background layer transition

(fixes part of #10748)

CHANGELOG.md

@@ -44,6 +44,7 @@ _Breaking developer changes, which may affect downstream projects or sites that
 * Add warning if aeroways cross each other, buildings or highways ([#9315], thanks [@k-yle])
 #### :bug: Bugfixes
 * Prevent degenerate ways caused by deleting a corner of a triangle ([#10003], thanks [@k-yle])
+* Fix briefly disappearing data layer during background layer tile switching transition ([#10748])
 #### :earth_asia: Localization
 #### :hourglass: Performance
 #### :mortar_board: Walkthrough / Help

css/80_app.css

@@ -4449,6 +4449,15 @@ img.tile-debug {
     bottom: 0;
 }
 
+.layer-background {
+    z-index: 1;
+}
+.layer-overlay {
+    z-index: 2;
+}
+.layer-data {
+    z-index: 3;
+}
 
 /* Map-In-Map
 ------------------------------------------------------- */