Just for clarity to ensure I'm understanding the problem.

The destination folder 'A' contains subfolders. The destination folder is in a collapsed state so you do not see it's subfolders. You are moving a folder 'B' to the destination folder 'A' You want to just drop folder 'B' into destination folder 'A' You do not need to expand destination folder as you are just dropping into it.

But everytime you hover over destination folder 'A' and just about to drop 'B' folder, the detination folder suddenly changes from a collapsed state to an expanded state, which in your case moves the destination folder position and causes you an issue where you may end up dropping folder into a subfolder or another folder instead.

Problem did not exist in version 102.15.1.

But what version of Thunderbird are you using now And what OS are you using ?

Previously in version 115* I recall it opened way too quickly and as you dragged folder down to destination folder, it immediately opened everything you passed over - not funny. So the latest version has all the timings slowed down.

I'm using Windows 10 OS and TB version 128.2.3 Testing: I tried to move a folder to my collapsed state 'Archives' folder which has many subfolders. If I move and drop reasonably quickly then it does not expand, but if I hover for a short while to be certain of the correct drop then it does auto expand. So I'm finding a definite pause and if I keep dragging over and beyond that 'Archives' folder it never expands.

So in my caes it depends upon the speed and whether I pause over 'Archives' for a while to force an auto expand - done because some people did not want to manually open a collapsed folder before performing the drag and drop.

There is the another option: Right click on folder > 'Move to' > select account and then select folder.

If you want to move another one to same folder: Right click on folder > 'Move to' > you should see the last folder named at bottom of list, so quicker to select.

Mind you the latest version now allows you to select more than one folder at a time, so you can move eg: 4 in one go to same destination.

Toad-Hall, thanks. Close, but not exactly. You are entirely correct with respect to Folder 'A' and its subfolders, and that I want to move Folder 'B' into 'A'. But, no, I don't hover long enough over 'A' that it expands before I drop B into it; instead, I manage to drop 'B' into the collapsed 'A' but then 'A' invariably expands or opens when I release the mouse button upon finishing the drop. This shouldn't happen, and is highly inconvenient.

Now using TB 115.10.1 on Windows 11 Pro with full Admin rights.

Sounds like bug 1863845 https://bugzilla.mozilla.org/show_bug.cgi?id=1863845

But that says fixed in Thunderbird 115.5.2