Use twm instead of fluxbox, as fluxbox insists on drawing a background, and any configuration attempt fails half of the time.

.travis.yml

@@ -4,7 +4,7 @@ addons:
     packages: &common_apt_packages
       # graphical environment
       - scrot
-      - fluxbox
+      - twm
       # automated GUI testing
       - sikuli-ide
       # Missing dependencies for sikuli-ide

utils/gui-wrapper.sh

@@ -5,7 +5,8 @@ set -e
 resolution="$1" # e.g. 800x600x24 (width x height x bits_per_pixel)
 shift           # the following arguments are the program to execute and its arguments
 
-bg="$(mktemp --suffix='.png')"
+bg="$(mktemp --suffix='.xbm')"
+twm_cfg="$(mktemp --suffix='_twm.cfg')"
 anim="$(mktemp -d)"
 
 # Create solid black background
@@ -14,7 +15,13 @@ convert -size "$(echo "$resolution" | cut -d 'x' -f1-2)" \
         -auto-level +level-colors 'gray(192),gray(128)' \
         "$bg"
 
-xvfb-run -a --server-args="-screen 0 ${resolution}" sh -c 'fluxbox 2>/dev/null & sleep 5; fbsetbg -f "'"$bg"'"; sleep 5; utils/screenshots-loop.sh "'"$anim"'" & "$@"' utils/gui-wrapper.sh-subshell "$@"
+cat > "$twm_cfg" <<EOF
+RandomPlacement
+EOF
+
+# -fg chocolate -bg coral looks nice too :)
+echo "$bg $twm_cfg $anim"
+xvfb-run -a --server-args="-screen 0 ${resolution}" sh -c 'twm -f "'"$twm_cfg"'" & xsetroot -bitmap "'"$bg"'" -fg gray75 -bg gray50; sleep 1; utils/screenshots-loop.sh "'"$anim"'" & "$@"' utils/gui-wrapper.sh-subshell "$@"
 
 touch "$anim/stop-screenshots"
 for i in `seq 60`; do if test -e "$anim/anim-done"; then break; fi; sleep 1; done