[gdb/tui] Show regs when switching to regs layout

When starting gdb in CLI mode, running to main and switching into the TUI regs
layout:
...
$ gdb -q a.out -ex start -ex "layout regs"
...
we get:
...
+---------------------------------+
|                                 |
| [ Register Values Unavailable ] |
|                                 |
+---------------------------------+
...

Fix this by handling this case in tui_data_window::rerender.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR tui/28600
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28600

gdb/testsuite/gdb.tui/regs.exp

@@ -41,10 +41,12 @@ Term::command "layout regs"
 Term::check_box "register box" 0 0 80 8
 Term::check_box "source box in regs layout" 0 7 80 8
 
-set text [Term::get_line 1]
-# Just check for any register window content at all.
-Term::check_contents "any register contents" "\\|.*\[^ \].*\\|"
-
+# The current frame is main, check that registers are available.
+set re_reg_vals_unavailable \
+    [string_to_regexp {[ Register Values Unavailable ]}]
+gdb_assert \
+    { ![Term::check_region_contents_p 0 0 80 8 $re_reg_vals_unavailable] } \
+    "Register values available"
 
 # Check that we can successfully cause the register window to appear
 # using the 'tui reg next' and 'tui reg prev' commands.

gdb/tui/tui-regs.c

@@ -192,7 +192,7 @@ tui_data_window::show_registers (const reggroup *group)
       m_regs_content.clear ();
     }
 
-  rerender ();
+  rerender (false);
 }
 
 
@@ -415,10 +415,18 @@ tui_data_window::erase_data_content (const char *prompt)
 /* See tui-regs.h.  */
 
 void
-tui_data_window::rerender ()
+tui_data_window::rerender (bool toplevel)
 {
   if (m_regs_content.empty ())
-    erase_data_content (_("[ Register Values Unavailable ]"));
+    {
+      if (toplevel && has_stack_frames ())
+	{
+	  frame_info_ptr fi = get_selected_frame (NULL);
+	  check_register_values (fi);
+	}
+      else
+	erase_data_content (_("[ Register Values Unavailable ]"));
+    }
   else
     {
       erase_data_content (NULL);

gdb/tui/tui-regs.h

@@ -75,7 +75,11 @@ protected:
   {
   }
 
-  void rerender () override;
+  void rerender (bool toplevel);
+  void rerender () override
+  {
+    rerender (true);
+  }
 
 private: