Dom renderer: top decorations above selection

Fixes #3799

css/xterm.css

@@ -183,3 +183,8 @@
     right: 0;
     pointer-events: none;
 }
+
+.xterm-decoration-top {
+    z-index: 2;
+    position: relative;
+}

src/browser/renderer/dom/DomRendererRowFactory.ts

@@ -195,6 +195,11 @@ export class DomRendererRowFactory {
         isTop = d.options.layer === 'top';
       }
 
+      // If it's a top decoration, render on above the selection
+      if (isTop) {
+        charElement.classList.add(`xterm-decoration-top`);
+      }
+
       // Foreground
       switch (fgColorMode) {
         case Attributes.CM_P16: