[Lens] Show loading icon for suggestions immediately

[timroes]

Currently when changing the configuration the suggestion panels will still show the old suggestions until the new ones has been rendered. We should immediately switch into showing a loading state (e.g. Gray bars) as soon as the configuration has changed.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

I agree with this issue, and previously tried to get this behavior to happen while making changes to the ReactExpressionRenderer component, since I added the loading states in the first place. The reason we don't have this behavior is that we are using events from the search source, which introduces an artificial delay, and the artificial delay is what you are noticing.

The potential solutions are:

• Stop using ReactExpressionRenderer, replace it with something that we control
• Change ReactExpressionRenderer to show a loading spinner before the network request starts. @streamich previously pushed back on this change, when I tried it.
• Look for a way to remove the fetchSoon behavior which introduces artificial delay to requests. @lukasolson or @elastic/kibana-app-arch would be best for this.

[lukasolson]

Currently, when courier:batchSearches is set to true, we introduce this delay to batch up searches in a single _msearch call. That will be going away entirely in 8.0 (see #55140).

In the short term, we are planning on isolating fetchSoon and the branch of code relating to _msearch and courier:batchSearches and removing the delay when it is not set to true (see #61739).

[wylieconlon]

@lukasolson Thanks for the clarification. The work you've described of moving fetchSoon into a branch of the code should solve this issue as well.

[flash1293]

I think the very noticeable delay here isn't caused by the fetch delay, but by the debouncing of the suggestions (which makes sense) and using index as key: https://github.com/flash1293/kibana/blob/4a1b05c843c39beb3ea2d4db35f444e4b120c428/x-pack/plugins/lens/public/editor_frame_service/editor_frame/suggestion_panel.tsx#L329

By using a more unique key here, we should be able to improve the situation a lot.

[flash1293]

Fixed in master - it's now immediately showing a loading bar on user interaction, updating the chart once the data arrives