Turns out Weston takes an X grab while delivering button events. Thus, trying to take a grab in response to a button press is futile.
850143b Inspector: Make picking work without a grab
gtk/inspector/inspect-button.c | 12 +++++-------
gtk/inspector/window.h | 2 ++
2 files changed, 7 insertions(+), 7 deletions(-)
Upstream: git.gnome.org