How to group panels in tabbed mode when they are in fractal mode?

Solution Details

When a panel is grouped in fractal mode with other panels and you would like to group it with these panels but in tabbed mode, please do the following:

1. Click and hold the left mouse button on the title of the panel that you would like to group with others. It is important to click on the text of the panel's title not a free space of panel's title bar. Otherwise a whole group of the panels will be moved.
2. Drag the panel away from the others until it is free floating, then release the left mouse button.
3. Click and hold the left mouse button on the title of the panel again, this time drag it back to the middle of the group where you want it. (Do not land on one of the four arrows at the edges.)
4. When the entire area of the panel that you would like to group the panel being moved with is highlighted in blue, release the left mouse button.

After that, you can change location of the panel in the tabbed group by dragging its tab label.

If this didn't work for you, you may need to try Preferences ► System ► View ► Desktop section ► Reset button.

This Knowledge Base Article discusses how to prevent panels from getting stacked in fractal mode with AD20 and newer:
Panels get rearranged i.e. stacked

This documentation on Working with Panels in Altium Designer may be of further interest:
https://www.altium.com/documentation/altium-designer/working-with-panels-in-altium-designer