Qt applications give a blank white window or segfault in TigerVNC

Qt applications fail to start up correctly in TigerVNC 1.7.0+dfsg-8ubuntu2 with fvwm 1:2.6.7-3 on an Ubuntu 18.04 system.

The Qt version is really 5.9.5+dfsg-0ubuntu2.5.

The problem is the window that (usually) comes up is solid white - no widgets are displayed.  The rest of the time, the application segfaults.

keepassxc gives a white window, and seems puzzled about the display resolution.

A simple Qt5 hello world also gives a white window.

konsole segfaults.

The hello world looks like:

#include <QtWidgets/QApplication>
 #include <QtWidgets/QPushButton>

int main( int argc, char **argv )

{ QApplication a( argc, argv ); QPushButton hello( "Hello world!", 0 ); hello.resize( 100, 30 ); hello.show(); return a.exec(); }

The corresponding Makefile looks like:

CPPFLAGS=-I/usr/include/x86_64-linux-gnu/qt5 -fPIC
 LDFLAGS=-lQt5Core -lQt5Widgets

go: qt-hello-world
 ./qt-hello-world

qt-hello-world: qt-hello-world.o
 g++ -o qt-hello-world qt-hello-world.o ${LDFLAGS}

I'm attaching to png's of what the hello world (hello-world.png) and keepassxc (keepassxc.png) look like, as well as the tty output from keepassxc (keepassxc.txt).

Thanks!