Fix TUI text centering

In a couple of spots, the TUI tries to center some text in the window.
Andrew noticed that the calculation is done strangely and the text
ends up somewhat to the left of center.

This patch fixes the problem.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31355

gdb/testsuite/gdb.tui/regs.exp

@@ -46,7 +46,10 @@ Term::check_box "source box in regs layout" 0 7 80 8
 
 # The current frame is main, check that registers are available.
 set re_reg_vals_unavailable \
-    [string_to_regexp {[ Register Values Unavailable ]}]
+    [string_to_regexp \
+	 [string cat \
+	      [string repeat " " 23] \
+	      {[ Register Values Unavailable ]}]]
 gdb_assert \
     { ![Term::check_region_contents_p 0 0 80 8 $re_reg_vals_unavailable] } \
     "Register values available"

gdb/tui/tui-data.c

@@ -169,13 +169,23 @@ tui_win_info::set_title (std::string &&new_title)
 /* See tui-data.h.  */
 
 void
-tui_win_info::display_string (int y, int x, const char *str) const
+tui_win_info::center_string (const char *str)
 {
-  int n = width - box_width () - x;
-  if (n <= 0)
-    return;
+  werase (handle.get ());
+  check_and_display_highlight_if_needed ();
 
-  mvwaddnstr (handle.get (), y, x, str, n);
+  int avail_width = width - box_size ();
+  int len = strlen (str);
+
+  int x_pos = box_width ();
+  if (len < avail_width)
+    x_pos += (avail_width - len) / 2;
+
+  int n = avail_width - x_pos;
+  gdb_assert (n > 0);
+
+  mvwaddnstr (handle.get (), height / 2, x_pos, str, n);
+  refresh_window ();
 }
 
 /* See tui-data.h.  */

gdb/tui/tui-data.h

@@ -167,9 +167,9 @@ public:
   const std::string &title () const
   { return m_title; }
 
-  /* Display string STR in the window at position (Y,X), abbreviated if
-     necessary.  */
-  void display_string (int y, int x, const char *str) const;
+  /* Clear the window, maybe draw the boarder, and then display string
+     STR centered in the window, abbreviated if necessary.  */
+  void center_string (const char *str);
 
   /* Display string STR in the window at the current cursor position,
      abbreviated if necessary.  */

gdb/tui/tui-regs.c

@@ -354,17 +354,7 @@ tui_data_window::first_data_item_displayed ()
 void
 tui_data_window::erase_data_content ()
 {
-  werase (handle.get ());
-  check_and_display_highlight_if_needed ();
-
-  const char *prompt = _("[ Register Values Unavailable ]");
-  int half_width = (width - box_size ()) / 2;
-  int x_pos;
-  if (strlen (prompt) >= half_width)
-    x_pos = 1;
-  else
-    x_pos = half_width - strlen (prompt);
-  display_string (height / 2, x_pos, prompt);
+  center_string (_("[ Register Values Unavailable ]"));
 }
 
 /* See tui-regs.h.  */

gdb/tui/tui-winsource.c

@@ -222,23 +222,9 @@ tui_update_source_windows_with_line (struct symtab_and_line sal)
 void
 tui_source_window_base::do_erase_source_content (const char *str)
 {
-  int x_pos;
-  int half_width = (width - box_size ()) / 2;
-
   m_content.clear ();
-  if (handle != NULL)
-    {
-      werase (handle.get ());
-      check_and_display_highlight_if_needed ();
-
-      if (strlen (str) >= half_width)
-	x_pos = 1;
-      else
-	x_pos = half_width - strlen (str);
-      display_string (height / 2, x_pos, str);
-
-      refresh_window ();
-    }
+  if (handle != nullptr)
+    center_string (str);
 }
 
 /* See tui-winsource.h.  */
@@ -714,7 +700,7 @@ tui_source_window_base::update_exec_info (bool refresh_p)
       if (src_element->is_exec_point)
 	element[TUI_EXEC_POS] = '>';
 
-      display_string (i + box_width (), box_width (), element);
+      mvwaddstr (handle.get (), i + box_width (), box_width (), element);
 
       show_line_number (i);
     }