Consider this gdb session:
...
$ gdb -q
(gdb) define foo
Type commands for definition of "foo".
End with a line saying just "end".
>bar
>end
(gdb) show user
User command "foo":
  bar

(gdb) 
...

Now let's try the same in TUI:
...
(gdb) define foo   
Type commands for definition of "foo".
End with a line saying just "end".
(gdb) show user
User command "foo":
  bar

(gdb) 
...

The secondary prompt lines are not terminated with a \n, and are consequently overwritten.

With this tentative patch:
...
diff --git a/gdb/top.c b/gdb/top.c
index 90ddc5f5ea7..f9b967c5c64 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -77,6 +77,7 @@
 
 #if defined(TUI)
 # include "tui/tui.h"
+# include "tui/tui-io.h"
 #endif
 
 #ifndef O_NOCTTY
@@ -958,6 +959,9 @@ gdb_readline_wrapper_line (gdb::unique_xmalloc_ptr<char> &&line)
   saved_after_char_processing_hook = after_char_processing_hook;
   after_char_processing_hook = NULL;
 
+  if (tui_active)
+    tui_inject_newline_into_command_window ();
+
   /* Prevent parts of the prompt from being redisplayed if annotations
      are enabled, and readline's state getting out of sync.  We'll
      reinstall the callback handler, which puts the terminal in raw
...
we get instead:
...
(gdb) define foo
Type commands for definition of "foo".
End with a line saying just "end".
>bar
>end
(gdb) show user
User command "foo":
  bar

(gdb) 
...