When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from outside Edebug (see Window Configurations). When you exit Edebug, it restores the previous window configuration.
Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the outside configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption.
Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs).
edebug-save-windows is non-nil (see Edebug Options).
If the value of edebug-save-windows is a list, only the listed
windows are saved and restored.
The window configuration is not restored on error or quit, but the
outside selected window is reselected even on error or quit in
case a save-excursion is active.
The window start and horizontal scrolling of the source code buffer are not restored, however, so that the display remains coherent within Edebug.
Saving and restoring the outside window configuration can sometimes
change the positions of point in the buffers on which the Lisp program
you are debugging operates, especially if your program moves point.
If this happens and interferes with your debugging, we recommend to
set edebug-save-windows to nil
(see Edebug Options).
edebug-save-displayed-buffer-points is non-nil.
overlay-arrow-position and
overlay-arrow-string are saved and restored, so you can safely
invoke Edebug from the recursive edit elsewhere in the same buffer.
cursor-in-echo-area is locally bound to nil so that
the cursor shows up in the window.