Saving here the part of JDK-8148360 which isn't covered by the fix provided for JDK-8142966.
It does not relate to hidpi as the issue is also reproducible at least with JDK8 b132 (and there is no need to set GDK_SCALE to reproduce).

JDK9 b101 + Ubuntu 14.04 Linux
Please run the 'FontTest.java' example from JDK-8148484 with different L&Fs (GTK, Metal, Motif).

there are the following issues (please see the screenshots):
1. the text somewhere isn't drawn fully
2. invalid selection highlighting
(and not sure if these two are bugs, but just in case)
3. the fonts (especially italic) are rough for Motif in comparison with other L&Fs (see 'motif.png')
4. sometimes the fonts are quite different from their HTML twins (see 'metal-2.png' for Courier)