Wow! It's down to line 4292 in MainWindow.cpp.
Code:
web_settings->setAttribute(QWebEngineSettings::LocalContentCanAccessFileUrls, false;
Changing that to true allows things (images, css, etc) to work again. But if that's the case, I'm not certain how loading local file url resources was
ever working in Sigil. That setting has been set to false since the first official version with QtWebEngine!
Perhaps they've finally gotten around to honoring the setting in Qt5.15, where earlier versions of Qt didn't?
@KevinH: Are there specific reasons why we've set that to false? The description of the attribute seems fairly self explanatory. And by my reckoning, we've
always needed local content to be able to load local file urls in Sigil, right?
Now if I can just figure out why my hardcoded dark theme got borked by 5.15.0