Description

Currently the file browser sets its own default config here:

Since this happens at application startup, it can override other configuration set in different plugins.

Reproduce

In a separate plugin (for example in a custom third-party extension), use browser.showFileCheckboxes = true to show the file browser checkboxes by default on application startup.

However because of the current logic with the code snippet above the default configuration will be applied again when the plugin activates.

In the following screencast we see how the checkboxes first appear and then quickly disappear:

Expected behavior

Other plugins should be allowed to use public setters like browser.showFileCheckboxes = true to customize the behavior of the file browser, when there is no user setting available.

If there is a value in the settings, then it takes precedence.

Context

This is an issue in Notebook 7 which has to use the settings to store its default values for the file browser (checkboxes, file size column, sort notebooks first). For example: jupyter/notebook#6892

However since these settings are also shared with JupyterLab, they also apply in the JupyterLab interface.

The idea would be for Notebook 7 to be able to customize with the following without having the configuration being overridden by the default lab plugin:

browser.showFileCheckboxes = true
browser.sortNotebookFirst = true
browser.showFileSizeColumn = true