Fix Progress component

[clarasb]

Description

This PR fixes the behaviour of progress components by adding hidden support for the components and connecting it to callback execution state.

Progress components can now be hidden with hidden=True and shown via callback outputs such as:

Output("progress", "hidden")

When a server-side callback declares a progress component’s hidden property as an output, chartlets now shows that progress indicator immediately while the callback request is pending. Once the callback completes, the normal callback output is applied, typically hiding the progress component again.

Changes

• Added visible to the shared component state model.
• Added pending-callback progress handling in invokeCallbacks().
• Added a DEV_NOTES.md to explain the Progress logic.
• Added another demo panel, demonstrating the progress component.
• Other changes:
  • Fixed vega_test.py.

Demo Panel

xcube-viewer panels demo

[codecov]

Codecov Report

❌ Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.57%. Comparing base (f8cf975) to head (f7f5ac9).

Files with missing lines	Patch %	Lines
...ackages/lib/src/actions/helpers/pendingProgress.ts	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   92.53%   93.57%   +1.03%     
==========================================
  Files          90       91       +1     
  Lines        1594     1665      +71     
  Branches      243      262      +19     
==========================================
+ Hits         1475     1558      +83     
+ Misses         99       91       -8     
+ Partials       20       16       -4     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)
frontend	87.70% <98.55%> (+2.55%)	⬆️

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[forman]

The implementation, especially the tests look overly complicated to me for what we want to achieve (display a progress indicator). But that may be due to my limited time.
I was not able to grasp what is done in pendingProgress.ts and what is actually tested in invokeCallbacks.test.ts.
And is the user supposed to set the visible flag? Why?

With that extra amount of code to be maintained it makes a lot sense to clearly document what has been developed here.

• Add a developer note (.md) about the current progress implementation
• Add docstrings to explain purpose and logic
• Comment tests

And

• Change name and meaning of property visible into hidden.

[forman]

visible is actually not a good choice as a component property. Many boolean component properties are optional and their default is therefore "falsy" (e.g. disabled). Hence this property should be called hidden, not visible.

[clarasb]

done.