Attempted config merge of list and dict clobbers dict

[vidartf]

Description

In the lab handler, we attempt to merge the page_config_data set on the webapp with the readout from the settings dir:

jupyterlab_server/jupyterlab_server/handlers.py

Line 124 in 52f4cf9

recursive_update(page_config, get_page_config(labextensions_path, settings_dir, logger=self.log))

The code that reads out data from the config file includes a part that converts dicts to lists:

jupyterlab_server/jupyterlab_server/config.py

Lines 170 to 174 in 3c7a83c

	# Convert dictionaries to lists to give to the front end
	for (key, value) in page_config.items():
	if isinstance(value, dict):
	page_config[key] = [subkey for subkey in value if value[subkey]]

However, this is run before the attempted merge with serverapp data. So, if someone has set dict data for a key (e.g. "disabledExtensions" on the serverapp data, it gets clobbered by the (potentially empty) list from the settings dir.

Expected behavior

The conversion from dict to list happens after all merging has happened. As recursive_update does not merge lists, this seems the most reasonable option.

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[tfmark]

Is this related to this change ? we have pinned jupyterlab-server==2.6.0 because our overrides.json was getting squashed.