LXQt Arc theme + Breeze widgets hides menu scroll bar when searching

[ԜаӀtеr Ⅼарсһуnѕkі]

STEPS TO REPRODUCE
 1. Click the menu
 2. While not highlighting any menu item, type a
 3. Mouse over the results
 4. Use the scroll wheel to move down a screen

Note there are additional results despite no scroll bar being displayed.

Two idiosyncracies regarding #2:
 1. This works in English. If you have another language, especially one that does not use Latin characters, you will certainly want to use a different character.
 2. The important thing is that you have a set of results that are greater than the maximum number of entries to display. To see that, right click on the menu button and click "Configure Application Menu" and then look at the value for "Maximum visible items," which defaults to 10.

EXPECTED RESULTS
A scroll bar should be visible. You'll notice one is if you use, for example, the Ambience LXQt theme.

ACTUAL RESULTS
The scroll bar is entirely invisible.

NOTES
Looks like this is in Ambience:

#MainMenu QScrollBar::handle {
    background-color: palette(highlighted-text);
    color: palette(highlighted-text);
}

but there does not seem to be something similar in our theme.

The default for upstream is Frost and it also suffers the same problem.

AFFECTED VERSIONS
Lubuntu 18.10, lubuntu-artwork 1.6, lxqt-panel 0.13.0-0ubuntu5
Lubuntu 19.04, lubuntu-artwork 19.04.2, lxqt-panel 0.14.1-0ubuntu1