Collab panel touch ups (#2855)

This will also fix the bug that @JosephTLyons observed where accepting a
channel invite would not show sub channels.

Release Notes:

- Offline section is now collapsed by default
- Manage members now shows full list
- Dragging of docks now follows the mouse exactly, and double clicks
reset size. (https://github.com/zed-industries/community/issues/1816)

styles/src/style_tree/collab_modals.ts

@@ -76,7 +76,8 @@ export default function channel_modal(): any {
                 },
 
             },
-            max_height: 400,
+            // FIXME: due to a bug in the picker's size calculation, this must be 600
+            max_height: 600,
             max_width: 540,
             title: {
                 ...text(theme.middle, "sans", "on", { size: "lg" }),