Bug 486196

SUMMARY
Dolphin doesn't remember the state of the folder panel per view (tab or split view).

There are two problems with that:
* Collapsing folders when switching tabs/views essentially foils the advantage and, in my opinion, the main use case of tabs/split views.
* Performance: the issue is particularly pronounced if one of the locations is sftp:// over the internet, because every time you switch to the sftp tab/view, the directory structure is reloaded which often takes several seconds due to the slow ssh connection.

STEPS TO REPRODUCE
1. Open Dolphin in your home directory. If the folder panel is not shown, turn it on (F7).
2. Expand at least one directory in the folder panel by pressing the ">" left to the folder name in the folder panel. But do not enter a subdirectory.
3. Open a second tab or split the view. Navigate to a location that has a different folder panel root than home. The home directory in the first tab and the root directory in the second tab will do for illustration (but to really see how annoying this is, open an sftp location over the internet).
4. Switch back to the first tab/view.

OBSERVED RESULT
The folder panel gets reloaded (in case of sftp slowly) and its root correctly switches back to home. But all expanded folders are collapsed, except the one that is currently selected.

EXPECTED RESULT
The folder panel should remember its state per tab and split view. Note that one could use the same state for tabs/views that have the same folder panel root (e.g. the home directory), but not for the root directory and sftp locations.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE tumbleweed
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0