Allow RHP modals to be resized #103561

Problem

The RHP has a fixed width. Long content often doesn't fit. Some customers want the RHP to be much wider than we allow now.

Proposal

Let users drag a splitter on the left edge of the RHP to resize it, like resizing a thread panel in Slack. This builds on the floating (inset) RHP from feat: use floating RHP style on web (follow-up to [Design polish] Use floating RHP style).

Requirements

• Make the draggable bar easy to see on hover and while dragging, like Slack does.
• Snap back to the default width with an animation.
• Work on wide layouts (web/desktop).

Possible follow-up

Use the same pattern for the Concierge side panel. Users could then work with Concierge and their usual app content side by side without closing the panel.

Related

• [CFI] Allow table columns to be manually resized on web/desktop. Resizable table columns ship first. RHP resizing comes next.

Next steps

@war-in has a working POC. Let's settle the design tweaks on this issue.

Reported in Slack.