(I think this is obviously my fault and more a feature request for IDM. 
But nevertheless, if someone knows a way to avoid this problem (beside the hint: "Open your eyes and use your brain ..") it will be welcomed.)

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Usually I don't use the "side-by-side display" of different files. But now I stumbled about a display constellation which made me editing the wrong file. 

See attachment and

• open two files - the two file tabs are displayed.
• Display two files side by side.
• Now I was accustomed to look at the tabs for current file and thought (of course by mistake) that the left tab shows the left window.

File tab order versus file windows display after usage of command tile vertical

The command Tile Vertical used by you most likely is a feature of Microsoft's multi-document interface library (MDI). So I suppose there is no special code in UltraEdit for this command. The MDI ignores the file tab order as file tabs are not managed by the MDI itself. It uses the history order of the MDI windows. Let me explain it on a simple example.

There are opened currently the three files FileA.txt, FileB.txt and FileC.txt and displayed in that order on file tabs bar. None of them is minimized to bottom of MDI client area (document window workspace). The currently active file is FileC.txt. The previously active file was FileA.txt and the FileB.txt is the oldest on z-order which is the recently used time order.

On usage of Tile Vertical the MDI positions according to z-order on most left side FileC.txt being the active file. The middle area is used for FileA.txt. And right area of MDI client area is used for FileB.txt. The minimized document windows are ignored by MDI on executing this command.

The files are listed as used by the MDI on Tile Vertical in menu Windows with configuration setting Old-style window list not enabled at Advanced - Settings or Configuration - Application Layout - Advanced in older versions of UltraEdit (not found anymore in UE v24.20 in configuration).

A different behavior for tiling the document windows vertically on client area would require a complete re-implementation of this library command in code of UltraEdit.

But there is also the file tabs (document window) mode with not dockable file tabs bar. This mode can be enabled for example by right clicking on a file tab and left clicking in context menu in submenu File tabs on Dockable to toggle off the dockable file tabs mode. Now the file tab of your choice can be dragged and dropped to left/right side of always full sized document windows to split the workspace and show the files side by side (or above/below) each other.

See also the power tip Vertical & Horizontal Split Window.