Don't scroll right when a tag value is selected

[Klumbumbus]

Follow-up from #14666
When the horizontal scroll bar is necessary then clicking on a tag value (or double clicking to change the value) scrolls to the right.
This is a bit annoying as it always hides at least parts of the keys.

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2019-01-11 21:47:35 +0100 (Fri, 11 Jan 2019)
Build-Date:2019-01-12 02:32:14
Revision:14674
Relative:URL: ^/trunk

Identification: JOSM/1.5 (14674 en) Windows 10 64-Bit
OS Build number: Windows 10 Pro 1803 (17134)
Memory Usage: 791 MB / 1820 MB (134 MB allocated, but free)
Java version: 1.8.0_191-b12, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
Screen: \Display0 1680x1050
Maximum Screen Size: 1680x1050
VM arguments: [-Djava.security.manager, -Djava.security.policy=file:<java.home>\lib\security\javaws.policy, -DtrustProxy=true, -Djnlpx.home=<java.home>\bin, -Djnlpx.origFilenameArg=%UserProfile%\Desktop\josm-latest.jnlp, -Djnlpx.remove=false, -Djava.util.Arrays.useLegacyMergeSort=true, -Djnlpx.heapsize=NULL,2048m, -Djnlpx.splashport=56117, -Djnlpx.jvm=<java.home>\bin\javaw.exe]
Dataset consistency test: No problems found