Steps to reproduce:

• run the attached "window_flags.pro" project
• tick on the "Frameless" checkbox, click "Apply"
• tick off the "Frameless" checkbox, click "Apply"
• tick on the "Frameless" and "Transparent For Input" checkboxes, click "Apply"

Expected result: "window-flags-good.jpg"

Actual result: "window-flags-bad.jpg"

If you will tick on the "Frameless" checkbox and click "Apply", then tick on the "Transparent For Input" checkbox and click "Apply", then it also works as expected.

It's essential to have the following lines:

        setWindowOpacity(0.9f);
        setAttribute(Qt::WA_TranslucentBackground);