Split Layout: collapsible

[bendera]

Example: https://opensource.adobe.com/spectrum-web-components/storybook/index.html?path=/story/split-view--horizontal-resizable-collapsible

[IanMayo]

Hi, I'm also seeking this feature - in order to produce a web-view based activity bar. Use of vscode-elements would allow me to deliver this via a single webview component, instead of 4.

I'm aware of the Collapsible and SplitLayout components, but they don't exhibit the greedy behaviour we see in vs-code.