Hi alexalekhin
Thanks for searching for open issues. It helps us tremendously with triaging issues.
>'Extended choice parameter' has property 'Number of Visible Items' - is it possible to introduce similar to Active Choices?
I think so!
>So is it possible to change priority to this ticket and implement this feature?
I rarely look at priorities here in JIRA. As I am currently working on something else (Python & weather forecast models orchestration, completely unrelated to active choices) I don't have any $work time to work on active choices. So it's 100% volunteer/spare time.
If you are able to provide a pull request, it'd be faster to review, discuss, release. We had a meeting this week to discuss what issues we must include in the next release, which we are planning to prepare soon. Can't promise to include this one, but if I have spare time I can try 
Thanks!
Bruno
Code changed in jenkins
User: Bruno P. Kinoshita
Path:
src/main/java/org/biouno/unochoice/AbstractCascadableParameter.java
src/main/java/org/biouno/unochoice/AbstractScriptableParameter.java
src/main/java/org/biouno/unochoice/CascadeChoiceParameter.java
src/main/java/org/biouno/unochoice/ChoiceParameter.java
src/main/java/org/biouno/unochoice/DynamicReferenceParameter.java
src/main/resources/org/biouno/unochoice/CascadeChoiceParameter/config.jelly
src/main/resources/org/biouno/unochoice/ChoiceParameter/config.jelly
src/main/resources/org/biouno/unochoice/DynamicReferenceParameter/config.jelly
src/main/resources/org/biouno/unochoice/common/checkboxContent.jelly
src/main/resources/org/biouno/unochoice/common/choiceParameterCommon.jelly
src/main/resources/org/biouno/unochoice/common/radioContent.jelly
src/main/webapp/help-useDefaultMaxVisibleItemCount.html
src/main/webapp/help-visibleItemCount.html
src/test/java/org/biouno/unochoice/issue38889/TestNumberOfVisibleElements.java
src/test/java/org/biouno/unochoice/issue38889/package-info.java
http://jenkins-ci.org/commit/active-choices-plugin/83f53bd14f939711e56213ca91ad679fffedd7e6
Log:
JENKINS-38889 Add an option to disable the default maximum number of visible elements. Also add another option to specify the number of visible elements on the UI. Unit tests included.