Probably obvious, but not to this user.

How do I retrieve the title-bar of an app that was moved off the edge of the screen in X, in order that this app-window can be moved again to other screen locations??

I was working with several windows within Puppy (lupu-510), and had to move one or more of these windows around as I was carrying out different tasks.
E.g., Opened two or more windows for the urxvt terminal app, another one for the web browser, two or more for different file manager directories, another for the Geany text editor, ... etc.

I grabbed the title bar for one of the window apps (Geany text editor) and moved it to the top-edge of my monitor screen so I could have a little more central real-estate for the other apps I was using at the same time. Problem is, now I cannot get the title-bar of this app back in order to move this around as needed!

So again, How do I retrieve the title-bar of an app that was moved off the edge of the screen in X, in order that this app-window can be moved again to other screen locations as needed??

TYIA for further [clear] assistance on this

On most desktops (or maybe this is an X thing, generically?), pressing the Alt key while left-button-dragging with the mouse allows you to move a window. Is this what you meant?

--- rod.

Have you tried left-clicking on it's taskbar holder and selecting MOVE?

One way I resolved this was to use Puppy's Desktop --> Desktop Settings --> 'WM Switcher Switch between JWM and IceWM' app.
Doing this and switching back-and-forth between WM's restored all lost app-window titlebars (or this at least made certain no "disappeared" app-windows remained).

Another way this app-window disappearance resolved was to upgrade the minor version to lupu-511.
A 3rd way this problem can be avoided in the 1st place is to use more screen pages.

Is that different from a title-bar? If so, where is it? If not, then the original problem seems to be that the title bar isn't accessible for the purpose of clicking.

--- rod.

Yes it is! With the application open, you should see a representation of it on the task bar. That is the area between the Menu button and the time display at the bottom of the screen.
Right-click on that representation, select Move and then move the mouse pointer back to the desktop, and drag the outline of the application window where you want it to go.

Okay, that is a desktop add-on, and is probably very specific to the particular desktop in use (in KDE, there is the desktop pager). Given that the OP is using a fairly pedestrian window manager, this may not be present.

--- rod.