Fix formatter instability (#1911)

pkl-formatter/src/main/java/org/pkl/formatter/Builder.java

@@ -1467,7 +1467,10 @@ final class Builder {
     if (endsInLineComment(prev)) {
       return linesBetween(prev, next) > 1 ? TWO_NEWLINES : mustForceLine();
     }
-    if (hasTrailingAffix(prev, next)) return Space.INSTANCE;
+    if (hasTrailingAffix(prev, next)) {
+      if (isTerminal(prev, "(") && startsWithBlockComment(next)) return line();
+      return Space.INSTANCE;
+    }
     if (prev.type == NodeType.DOC_COMMENT) return mustForceLine();
     if (prev.type == NodeType.ANNOTATION) return forceLine();
     if (FORCE_LINE_AFFIXES.contains(prev.type) || next.type.isAffix()) {
@@ -1528,10 +1531,24 @@ final class Builder {
     return noNewlines ? ifNotWrap : new IfWrap(id, ifWrap, ifNotWrap);
   }
 
+  private static boolean startsWithBlockComment(Node node) {
+    var n = node;
+    while (n != null) {
+      if (n.type == NodeType.BLOCK_COMMENT) return true;
+      n = n.children.isEmpty() ? null : n.children.get(0);
+    }
+    return false;
+  }
+
   private static boolean hasTrailingAffix(Node node, Node next) {
     var n = next;
     while (n != null) {
-      if (n.type.isAffix() && node.span.lineEnd() == n.span.lineBegin()) return true;
+      // semicolons are dropped when formatting, so they never produce a trailing affix.
+      if (n.type.isAffix()
+          && n.type != NodeType.SEMICOLON
+          && node.span.lineEnd() == n.span.lineBegin()) {
+        return true;
+      }
       n = n.children.isEmpty() ? null : n.children.get(0);
     }
     return false;

pkl-formatter/src/test/files/FormatterSnippetTests/input/expr-if.pkl

@@ -31,3 +31,6 @@ foo6 =
 foo7 =
   if /* some comment */ (bar) 1
   else 2
+
+foo8 = if (
+  /* c */ bar) 1 else 2

pkl-formatter/src/test/files/FormatterSnippetTests/output/expr-if.pkl

@@ -41,3 +41,9 @@ foo7 =
     1
   else
     2
+
+foo8 =
+  if (/* c */ bar)
+    1
+  else
+    2