Drag and drop from nemo / caja to window in window group on taskbar is broken

 * Bug persists in both current stable mint 21.1 production version of Cinnamon and nemo or caja and Cinnamon version 5.6.7 and nemo 5.6.3 (while using mint daily builds ppa of now which I am running this moment). 
 * > inxi -G
Graphics:
  Device-1: Intel UHD Graphics 620 driver: i915 v: kernel
  Display: x11 server: X.Org v: 1.21.1.3 driver: X: loaded: modesetting
    unloaded: fbdev,vesa gpu: i915 resolution: 2560x1440
  OpenGL: renderer: Mesa Intel UHD Graphics 620 (KBL GT2)
    v: 4.6 Mesa 22.2.5
 * 64 bit
 * ~/.xsession-errors, or /var/log/syslog show nothing of interest in regards to this behavior as far as I can see

Related / overlapping to https://github.com/linuxmint/cinnamon/issues/11139

Issue

Drag-and-drop a file from a nemo or caja window to a window in a window-group
(such as two firefox instances) does not work, more specifically the window over whiches window-preview is hovered
does not come to foreground so one could drop the file into it.

Steps to reproduce

• install Linux Mint 21.1 x86_64
• open two firefox instances (or, probably, any other program that can have multiple windows under one icon in the taskbar)
• open a nemo or caja window
• drag a file from the nemo or caja window over the firefox taskbar icon (the window preview showing the two ff windows appears)
• the window does not come to foreground, so dropping is not possible / does not cause ff to accept the file

Expected behaviour

• hover over taskbar icon (previews appear)
• hover over one preview
• corresponding ff window comes to foreground and accepts dropped file

Other information

• This bug breaks a very central feature of a desktop OS
• It used to work perfectly under mint 20.x.
• Bug is kind of indeterministic, because sometimes I cannot reproduce it with caja and dragging and dropping between several caja windows does work.

Reproduce the drag from a nemo or caja window through a grouped taskbar icon using the steps in the issue, then trace Cinnamon's taskbar window-preview handling. Compare the behavior with the related issue 11139. Done means hovering a grouped window preview brings that window to the foreground and permits the file drop.