Make sure the aria-live element is hidden when printing

FIX: Fix an issue where, on some browsers, the screenreader announced text ended up
in the printed document.

Closes codemirror/dev#1298

src/editorview.ts

@@ -183,7 +183,7 @@ export class EditorView {
     this.scrollDOM.appendChild(this.contentDOM)
 
     this.announceDOM = document.createElement("div")
-    this.announceDOM.style.cssText = "position: fixed; top: -10000px"
+    this.announceDOM.className = "cm-announced"
     this.announceDOM.setAttribute("aria-live", "polite")
 
     this.dom = document.createElement("div")

src/theme.ts

@@ -140,6 +140,14 @@ export const baseTheme = buildTheme("." + baseThemeID, {
     display: "block"
   },
 
+  ".cm-announced": {
+    position: "fixed",
+    top: "-10000px"
+  },
+  "@media print": {
+    ".cm-announced": { display: "none" }
+  },
+
   "&light .cm-activeLine": { backgroundColor: "#cceeff44" },
   "&dark .cm-activeLine": { backgroundColor: "#99eeff33" },