From the specification:

"Returns whether the always-on-top mode is supported for this window. Some platforms may not support always-on-top windows, some may support only some kinds of top-level windows; for example, a platform may not support always-on-top modal dialogs.

Returns:
true, if the always-on-top mode is supported BY THE TOOLKIT and for this window, false, if always-on-top mode is not supported for this window or TOOLKIT doesn't support always-on-top windows."

But it is inexplicitly - what toolkit we have in mind. Maybe, DEFAULT toolkit?

The following conformance test will fail due to this issue:
api/java_awt/Window/indexTGF.html#isAlwaysOnTopSupported[isAlwaysOnTopSupported_negative_customToolkit]