Available templates datasource reacts on value context changes and triggers expensive computation

Steps to reproduce

• create component dialog
• create page dialog
• template column filter

Expected results

• the combobox is populated with options only once (as the template availability resolving might be expensive)

Actual results

• the combobox is repopulated with any value context change

Workaround

Patch for Magnolia 6.2.9

<dependency>
  <groupId>info.magnolia.pages</groupId>
  <artifactId>magnolia-pages-app</artifactId>
  <version>6.2.8-PAGES-334</version>
</dependency>

Development notes

• Roman mentioned here that "it shows available templates for the page in value context (or root when no selection)" This should not happen for list view.
• Same issue is with tree view, but we might be soon removing tree filtering altogether anyway. In case we would want to keep filtering in tree view, read this too, otherwise irrelevant: It's not desirable even in Tree view, b/c from user point of view, I am adding a filter on top of the grid and I would expect to be able to filter out everything in the grid in this way. The fact that there is a value context amended by current selection and that it affects filters is a very technical detail a user has no knowledge of.