## Summary
Closes https://github.com/elastic/kibana/issues/187587
This PR changes how the dashboard panel selection items get computed, it
had previously been computed eagerly, in this implementation panel
selection items would only be computed when the user actually clicks the
`add panel` button, with it's results cached so that subsequent
interactions with the `add panel` button leverages the already computed
data.
**Notable Mention:**
The options presented as the dashboard panel list now only comprise of
uiActions specifically registered with the uiAction trigger
`ADD_PANEL_TRIGGER` and specific dashboard visualisation types. See
https://github.com/elastic/kibana/pull/187797#discussion_r1681320456 to
follow the reasoning behind this.
That been said adding new panels to the dashboard, would be something
along the following lines;
```ts
import { ADD_PANEL_TRIGGER } from '@kbn/ui-actions-plugin/public';
uiActions.attachAction(ADD_PANEL_TRIGGER, <registredActionId>);
// alternatively
// uiActions.addTriggerAction(ADD_PANEL_TRIGGER, ...someActionDefintion);
````
### Visuals
7c029a64-2cd8-4e3e-af5a-44b6788faa45
### How to test
- Navigate to a dashboard of choice
- Slow down your network speed using your browser dev tools, refresh
your dashboard, and click on the “Add panel” button as soon as it is
available (before the panels have a chance to load).
- You should be presented with a loading indicator, that eventually is
swapped out for the list of panels available for selection.
### Checklist
Delete any items that are not applicable to this PR.
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- [x] [Unit or functional
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were updated or added to match the most common scenarios
- [x] Any UI touched in this PR is usable by keyboard only (learn more
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### Risk Matrix
Delete this section if it is not applicable to this PR.
Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.
When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:
| Risk | Probability | Severity | Mitigation/Notes |
|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces—unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes—Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |
### For maintainers
- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
-->
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