SUMMARY
using graphic tablet stylus doesn't move on image window. only can with mouse.
kde is have this issue other application too. kdenlive, and taskbar folder view. 
https://bugs.kde.org/show_bug.cgi?id=416421
but a case resolved for desktop effects window scroll bar. systemsettings5. so i think this issue is can solved too.
this issue open from qt5.12.4. fix plan is qt version 5.14.1 later.
https://github.com/linuxwacom/xf86-input-wacom/issues/93
https://bugreports.qt.io/browse/QTBUG-77826

Likely to be a workaround? write this code for .profile.
“QT_XCB_TABLET_LEGACY_COORDINATES=1”
or “export QT_XCB_TABLET_LEGACY_COORDINATES=1”
but, doesn't this method cannot fix this issue for ubuntu user. why? i don't know.
some difference opensuse or arch and ubuntu, maybe.
opensuse tumbleweed use qt 5.14.1.  but have this issue yet. so The cause of the problem is qt application side?

STEPS TO REPRODUCE
1.using graphic tablet stylus 
2. image open
3. zoom or 100%
4. click and move stylus.

OBSERVED RESULT
if using graphic tablet stylus on gwenview is can't move on image window with zoom or 100%

EXPECTED RESULT
can move with graphic tablet stylus on image window.

SOFTWARE/OS VERSIONS
graphic tablet: wacom tablet bt M
Operating System: openSUSE Tumbleweed 20200214
KDE Plasma Version: 5.18.0
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1
Kernel Version: 5.5.2-1-default
OS Type: 64-bit