diff --git a/src/xdisp.c b/src/xdisp.c
index 987c760c74..b20c7718bd 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -1419,6 +1419,33 @@ window_hscroll_limited (struct window *w, struct frame *f)
   return window_hscroll;
 }
 
+static bool
+current_tab_visible_p (struct window *w, int *tab_num)
+{
+  bool visible_p = false;
+
+  if (window_wants_tab_line (w))
+    {
+      struct it it;
+
+      Lisp_Object window_tab_line_format
+	= window_parameter (w, Qtab_line_format);
+
+      w->tab_line_height
+	= display_mode_line (w, TAB_LINE_FACE_ID,
+			     NILP (window_tab_line_format)
+			     ? BVAR (current_buffer, tab_line_format)
+			     : window_tab_line_format);
+
+      init_iterator (&it, w, -1, -1, NULL, TAB_LINE_FACE_ID);
+
+      fprintf (stderr, "tab_line_p=%d truncated_on_right_p=%d last_visible_x=%d\n",
+               it.glyph_row->tab_line_p, it.glyph_row->truncated_on_right_p, it.last_visible_x);
+    }
+
+  return visible_p;
+}
+
 /* Return true if position CHARPOS is visible in window W.
    CHARPOS < 0 means return info about WINDOW_END position.
    If visible, set *X and *Y to pixel coordinates of top left corner.
@@ -24897,6 +24924,13 @@ display_mode_lines (struct window *w)
 
   if (window_wants_tab_line (w))
     {
+      int tab_num = 0;
+
+      if (current_tab_visible_p (w, &tab_num))
+        {
+          /* Set some Lisp variable */
+        }
+
       Lisp_Object window_tab_line_format
 	= window_parameter (w, Qtab_line_format);
 
@@ -24984,7 +25018,10 @@ display_mode_line (struct window *w, enum face_id face_id, Lisp_Object format)
   it.glyph_row->full_width_p = true;
   it.glyph_row->continued_p = false;
   it.glyph_row->truncated_on_left_p = false;
-  it.glyph_row->truncated_on_right_p = false;
+
+  /* Currently only tab-line needs truncation detection.  */
+  if (face_id != TAB_LINE_FACE_ID)
+    it.glyph_row->truncated_on_right_p = false;
 
   /* Make a 3D mode-line have a shadow at its right end.  */
   face = FACE_FROM_ID (it.f, face_id);