kchmviewer is a CHM (Winhelp) files viewer written on Qt/KDE. It can be build as a standalone Qt-based application, or a KDE application. The main point of kchmviewer is compatibility with non-English chm files, including most international charsets.
build at a4a3984465cb635822953350c571950ae726b539 Run on Fedora 34 GNOME 40 Wayland with Qt 5.15.2
The window will just disappear when entered full screen mode. Press F11 and the window will come back.