Support virtualization in Code action list

[rebornix]

while working on a python/notebook file, language server suggests tons of code actions, which blocks the UI for ~5 seconds

[rebornix]

The UI freeze is caused by DOM reads

[justschen]

related to microsoft/pylance-release#3873

[mjbvz]

The list is already virtualized but this layout call forces the entire thing to render (before correctly shrinking it back down):

vscode/src/vs/platform/actionWidget/browser/actionList.ts

Line 247 in e9b6a77

this._list.layout(heightWithHeaders);

The layout call is used to find the max width of the items in the list and then resize the widget to fit this. I'm not sure what the best way to do this for a list is, but at the very least we could probably make a single call to getBoundingClientRect on the container node instead of on each element

[justschen]

After change from above PR

Blocking time down from: