[BUG] build from source - qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found about onthespot HOT 3 OPEN

There seems to be a missing dependency in the readme. I had your same issue and installing adwaita-qt pulled in a list of about 11 qt related packages (one of them is required), after rebuilding onthespot with them installed it, ran just fine.

from onthespot.

It was just by chance to be honest, I use onthespot whenever I launch spotify so that any songs I play are downloaded but was sick of the white ui. I had 2 hours free last night and coded in an option for a dark mode. The app would not compile and I knew that xcb is qt related so I installed qtcore and the app still would not compile. I installed adwaita-qt because I knew it would pull in all the required deps I was missing, after which the app compiled fine and I uninstalled adwaita-qt.

One of the following is required to build the app:

dev-libs/double-conversion-3.2.1
dev-qt/qtcore-5.15.12-r1
x11-libs/xcb-util-renderutil-0.3.10
x11-libs/xcb-util-wm-0.4.2
dev-qt/qtgui-5.15.12
dev-qt/qtdbus-5.15.12
dev-qt/qtwidgets-5.15.12
dev-qt/qtx11extras-5.15.12

from onthespot.

Thank you @just37 , it started successfully after installing adwaita-qt :)
Would you mind sharing details how you figured this out?

from onthespot.