I do not use UltraEdit for Mac, but maybe the following from UltraEdit for Windows helps nevertheless.

There are two document window respectively file tab modes:

1. Dockable tabs / windows makes each document window a separate window with a frame and a window title. If one of the document window is maximized, all other document windows are displayed also maximized. But if all document windows are displayed normally, the document windows with window frame and title can be freely arranged within the client area of the application window. Overlapping document windows and minimized document windows are also possible in this document window mode.
2. Not dockable tabs / windows results in the document windows are in general maximized within the client area of the application window without a window frame and a window title but it is possible to arrange two or more windows side by side horizontally or vertically. The UltraEdit Mac/Linux power tips contain the power tip Drag-N-Drop Split Pane Editing… Vertical/Horizontal Split Window which explains how the window arrangement can be done with this document window / file tabs mode.

It looks like not dockable tabs / windows is used by you at the moment.

In UltraEdit for Windows can be opened Advanced - Settings or Configuration - Application layout - File tabs - Operation and unchecked or checked the configuration setting Dockable tabs. Another possibility is opening the file tabs context menu by right clicking on any file tab and clicking in the context submenu File tabs on the first menu item Dockable to toggle this configuration setting and with it the document tabs / windows mode.

It is not possible with not dockable tabs / windows preventing splitting the client area of the application window and arrange two or more document windows side by side vertically or horizontally. If a user drags and drops the file tab of a document window by mistake to a new position for the document window, another drag and drop is necessary to turn off the splitting by moving the file tab respectively the document window back to previous position, i.e. the separate document window over the area of the other document windows. It would be possible to use dockable tabs / windows and maximize them to prevent an accidental document window splitting. There is shown the Open Files bar on using the dockable document window mode which takes some vertical space.

A drag and drop of a file tab can be used also to just change the order of the file tabs in the file tabs bar respectively the order of the document windows of the opened files. In this case the user must take care after left clicking on a file tab and holding the mouse button that the drag is done only left / right on the file tabs bar. There is immediately shown during dragging the file tab left / right on the file tabs bar where the file tab respectively the document window will be in the document window order as shown on the file tabs bar on leaving the mouse button. Changing the order of the document windows by drag and drop the file tabs on the file tabs bar can be used in both document window modes: dockable and not dockable.

Thank you for the screenshot. There are not much file tabs related configuration settings in Preferences of UltraEdit for Mac. I supposed that already because most of the features regarding to file tabs and document window management are from the Microsoft GDI+ library used by UltraEdit on Windows and control the parameters of this window framework. It looks like a document window dragged and dropped by mistake to a new window position must be dragged and dropped back to former position of the window, i.e. combine the split document window area back to a single document window area.

BTW: Workspace is in context of UltraEdit everything inside the document window area like which files are opened and in which order, the position and size of each document window, bookmarks and folds (hidden lines) in the opened files, position of caret in each opened file, and some other data regarding to the visible elements of a document window.
Layout (former name environment) is in context of UltraEdit everything within main application window with the exception of the workspace area, i.e. the menus and toolbars (or the ribbon), the docked or floating views, their positions, sizes, view specific options (invisible, permanently visible, visible with auto-hide, split view), etc.