The “Skip Download” popup displays incorrectly

[Mormegil]

What steps will reproduce the problem?

1. Start the current tested version of JOSM (9329) on Windows 10.
2. Plugins will try to auto-update
3. However, the current reverter plugin requires JOSM version 9514 (that is probably a bug by itself, offering to update a plugin to a version which does not support the current version of JOSM?).
4. JOSM now asks if it should download anyway or skip the download.

What is the expected result?

The dialog should be shown to the user.

What happens instead?

A tiny piece of the title bar “Skip dow…” appears as a new top-level window; the user has to resize the window manually to see its content with the buttons. And when the user switches to another application, switching back to the JOSM splashscreen hides the popup completely, showing just “Updating plugins” with the spinner bar moving back and forth. Until the user manages to Alt+Tab to the almost-invisible window, he/she is stuck.

Please provide any additional information below. Attach a screenshot if possible.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2016-01-06 17:30:31 +0100 (Wed, 06 Jan 2016)
Build-Date:2016-01-06 16:32:31
Revision:9329
Relative:URL: ^/trunk

Identification: JOSM/1.5 (9329 en) Windows 10 64-Bit
Memory Usage: 79 MB / 247 MB (16 MB allocated, but free)
Java version: 1.8.0_73, Oracle Corporation, Java HotSpot(TM) Client VM
VM arguments: [-Duser.home=C:\Users\***\PortableApps\JavaPortableLauncher\Data\AppData]

Plugins:
- RoadSigns (31895)
- reverter (31926)
- turnlanes (31772)
- turnrestrictions (31895)

Last errors/warnings:
- E: org.openstreetmap.josm.plugins.PluginDownloadException: Download skipped

[Mormegil]

Screenshot of the initial state

[Don-vip]

In 9866/josm:

fix #12559 - The “Skip Download” popup displays incorrectly