There is cursor on the page (one page - one text frame) in inDesign.
Is it possible with JavaScript to get number/id of current page (text frame) with cursor?
Probably solution app.activeWindow.activePage.name returns number of the page on the screen, but not number of page/text frame with cursor.
For example: if cursor set on page 3, but we see page 20 on the screen, app.activeWindow.activePage.name returns 20.
The activeWindow indeed is the page that is visible in your current view, and you can scroll away from the text cursor.
To get the page that a cursor is in, first get the object that contains the cursor (usually a text frame) and then read its parentPage property:
readonly The page on which this page item appears.
Untested but it should be as easy as
pagenr = app.selection[0].parentTextFrames[0].parentPage.name;
where app.selection[0] is the current selection (for a 'clicked but not selected' text cursor its length is always 1), the parentTextFrames[0] refers to the frame containing the text and cursor, and then it's a short hop up to the parentPage and its properties.