Use [official VS Code views](https://code.visualstudio.com/api/ux-guidelines/views)

[T3sT3ro]

Extracted (copy-pasted) from issue #705.

The view widget used in the sidebar in the current FM extension seem to differ from the regular view widget used in other extensions (e.g. the "Explorer" sidebar). The differences between FM's sections and Explorers sections are:

• FM's views don't animate when opened/closed
• FM's views are not tab/keyboard navigable (space to activate, arrow up/down to move focus)
• FM's views aren't separated with a horizontal divider (border?) while section in the Explorer are
• FM's views can't be reordered with drag and drop.

To address it: Use official VSCode views and follow their guidelines.