Problem/Motivation
Maintainers of distributions and themes have pointed out that there is some bleed through of css from themes into the settings tray, which can significantly impact experience and usability.
We did #2853208: [META] Determine best method ensure consistent theming of Off Canvas Tray
Which involved this issue and #2853222: Explore using an IFrame to sandbox CSS for the Off Canvas tray
It was determined that iframe though it would provide better CSS isolation it would introduce its own problems to solve.
Proposed resolution
Add an aggressive CSS reset to the off-canvas tray to the module that will to the extent possible offer a uniform look when using the tray across different themes.
No CSS reset will ever be absolute because we can't control the CSS used on a sites current theme.
Since we decided not do #2847522: Allow off-canvas links to be rendered by either "default" or "admin" renderer there does not need to be a different look for the off-canvas tray when used by the Settings Tray module and other uses of the tray.
This module provides 2 libraries drupal.off_canvas and drupal.outside_in.
The CSS reset and other CSS related to the tray should be contained within the CSS files of the drupal.off_canvas and should follow the off-canvas.*.css naming pattern.
The drupal.off_canvas library will be moved out of the Settings Tray module in #2784443: Move off-canvas functionality from Settings tray module into drupal.dialog.ajax library so that other modules can use it
The drupal.outside_in library should not contain CSS that deals with tray. It should contain CSS that deals with other functionality of the Setting Tray module such as toolbar, highlighting regions, etc.
Remaining tasks
Test the patch.
User interface changes
Theme styles should not bleed into the tray.
API changes
none
Data model changes
none