add option to fix header in scroll box
diff --git a/R/scroll_box.R b/R/scroll_box.R
index 3d1b805..b0660ed 100644
--- a/R/scroll_box.R
+++ b/R/scroll_box.R
@@ -8,6 +8,8 @@
 #' @param width A character string indicating the width of the box, e.g. "100px"
 #' @param box_css CSS text for the box
 #' @param extra_css Extra CSS styles
+#' @param fixed_thead A list of two named element. enabled and background.
+#' Default is F and white, e.g. "list(enabled = T, background = "#fff")"
 #'
 #' @export
 #'
@@ -26,23 +28,45 @@
 
 scroll_box <- function(kable_input, height = NULL, width = NULL,
                        box_css = "border: 1px solid #ddd; padding: 5px; ",
-                       extra_css = NULL) {
+                       extra_css = NULL, fixed_thead = list(enabled = F, background = "#fff")) {
+
   kable_attrs <- attributes(kable_input)
-  out <- as.character(kable_input)
+
+  if (fixed_thead$enabled) {
+    box_css = "border: 1px solid #ddd; padding: 0px; "
+    kable_xml <- read_kable_as_xml(kable_input)
+    kable_thead <- xml_tpart(kable_xml, "thead")
+    original_header_row <- xml_child(kable_thead, length(xml_children(kable_thead)))
+    for (theader_i in 1:length(xml_children(original_header_row))) {
+      target_header_cell <- xml_child(original_header_row, theader_i)
+      xml_attr(target_header_cell, "style") <- paste0(xml_attr(target_header_cell, "style"),
+                                                      "position: sticky; top:0; background: ",
+                                                      fixed_thead$background,";")
+    }
+    out <- as.character(as_kable_xml(kable_xml))
+  } else {
+    out <- as.character(kable_input)
+  }
+
   box_styles <- c(box_css, extra_css)
+
   if (!is.null(height)) {
     box_styles <- c(box_styles,
                     paste0("overflow-y: scroll; height:", height, "; "))
   }
+
   if (!is.null(width)) {
     box_styles <- c(box_styles,
                     paste0("overflow-x: scroll; width:", width, "; "))
   }
+
   out <- paste0('<div style="', paste(box_styles, collapse = ""), '">',
                 out, '</div>')
   out <- structure(out, format = "html",
                    class = "knitr_kable")
   attributes(out) <- kable_attrs
+
   if (!"kableExtra" %in% class(out)) class(out) <- c("kableExtra", class(out))
+
   return(out)
 }