Use CSS to control ViewContainerPart toolbar visibility

[colin-grant-work]

What it does

This PR supersedes #8967, as it contains alternative logic for updating the toolbar.

At the moment, we have quite an elaborate routine for managing the visibility of the toolbars on ViewContainerParts. The results differ from VSCode in a couple of ways:

• The toolbar disappears if a context-menu is opened using the ... more icon.
• The toolbar isn't always visible if the widget is focused.

How to test

1. Test different states of widget focus and hovering.
   • The toolbar should never be visible if the ViewContainerPart is collapsed
   • The toolbar should be visible if the widget is focused
   • The toolbar should be visible if you hover over the widget
   • The toolbar should be visible if a context menu is opened from the toolbar

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

Signed-off-by: Colin Grant [email protected]

[msujew]

Looks good me:

• Focusing the container part always shows the toolbar ✔️
• Hovering inside the container part always shows the toolbar ✔️
• A collapsed container part never shows the toolbar ✔️
• Using the ellipsis menu still shows the toolbar ✔️
• Everything works as expected when there's only a single container part inside the view ✔️

[vince-fugnitto]

The changes look good and work well for me 👍

• the toolbar should never be visible if the ViewContainerPart is collapsed
• the toolbar should be visible if the widget is focused
• the toolbar should be visible if you hover over the widget
• the toolbar should be visible if a context menu is opened from the toolbar
• confirmed that executing toolbar items, and menu items in the ... (more) context-menu works well