I run KDE Plasma (KWin window manager) on Gentoo Linux.

I have tested with several different builds of QCAD 3.27.8:

• Pro Trial qt5.14 (tar.gz distribution)
• Pro Trial (tar.gz distribution)
• Community Edition built by Gentoo Portage (https://github.com/l29ah/booboo/blob/master/media-gfx/qcad/qcad-3.27.8.0.ebuild)

If I use Open, Save As, and anything else that selects a file or a folder, the dialog does not show any contents, and the terminal prints an error:

Warning:  couldn't create slave: "Unknown protocol 'file'."

Some of the dialogs behave better by deselecting:
Application Prefs > File: Open/Save > File Dialog > Use system file dialog

So, looking through the code, I noticed that there is already KDE-specific handling to avoid using the system dialog, but it has probably been broken since switching to QT5, because Q_WS_X11 (and all other Q_WS_* global macros) no longer exist with QT5+.

I've attached a patch that fixes the usage of that macro. With that patch, in my portage-built Community Edition build, I no longer have to deselect "Use system file dialog".

But, there are at least two places that do not respect the DontUseNativeDialogs setting:

• Application Prefs > File > Templates > Template Folders > Add
• Application Prefs > Widgets > Library Browser > Library Sources > Add...

I found the scripts source for the Library Browser PreferencesPage.ui, but I couldn't find the same for the Templates selection, probably because that is a Pro-only feature (right?).

For the LibraryBrowser, here is the appropriate sections of code:

• The btAdd button on the DirectoryList: https://github.com/qcad/qcad/blob/master/scripts/Widgets/LibraryBrowser/PreferencesPage.ui#L135
• The DirectoryList's clicked function: https://github.com/qcad/qcad/blob/master/scripts/Widgets/DirectoryList/DirectoryList.js#L41

So, somehow this line needs to change to use the DontUseNativeDialogs setting, but I'm not sure how to do that (I don't have much experience with QT development).

var dir = QFileDialog.getExistingDirectory(this, fileDlgTitle);

Assuming that Template Folders preference also uses the DirectoryList widget, fixing this should fix that pref pane as well.

Warning:  couldn't create slave: "Unknown protocol 'file'."