Stabilize XML declaration formatting on reformat

src/printer.js

@@ -542,10 +542,16 @@ function printMisc(path, opts, print) {
 
 function printProlog(path, opts, print) {
   const { XMLDeclOpen, attribute, SPECIAL_CLOSE } = path.node;
-  const parts = [XMLDeclOpen];
+
+  // The XMLDeclOpen token includes the single whitespace character that follows
+  // "<?xml" (its pattern is /<\?xml[ \t\r\n]/). Printing that trailing
+  // whitespace verbatim is not stable: when the declaration breaks onto
+  // multiple lines a captured newline turns into an extra blank line on the
+  // next pass. Drop it and let the layout below supply the spacing.
+  const parts = ["<?xml"];
 
   if (attribute) {
-    parts.push(indent([softline, join(line, path.map(print, "attribute"))]));
+    parts.push(indent([line, join(line, path.map(print, "attribute"))]));
   }
 
   return group([

test/format.test.js

@@ -104,3 +104,13 @@ test("xmlQuoteAttributes => double", async () => {
 
   expect(formatted).toMatchSnapshot();
 });
+
+test("wrapped XML declaration is stable on reformat", async () => {
+  const input = '<?xml version="1.0" encoding="UTF-8"?>\n<root />\n';
+  const opts = { printWidth: 20 };
+
+  const once = await format(input, opts);
+  const twice = await format(once, opts);
+
+  expect(twice).toEqual(once);
+});