In Qt 6, <img src="file:///..."> the file URL is magled by QTextDocument when trying to load the resource. The URL passed to loadResource() is the original URL with the leading file:/ stripped. QUrl.isRelative() returns True for such URLs causing loadResource() to fail on Windows. On Unix it still works, because the leading double slash is equivalent to a leading single slash. On Windows, file:///C:/some/path becomes //c/some/path which fails to load.

A trivial PyQt6 script to reproduce the issue:

import os
from PyQt6.QtCore import QUrl
from PyQt6.QtGui import QTextDocument, QPainter, QImage
from PyQt6.QtWidgets import QApplication

class D(QTextDocument):
    def loadResource(self, rtype, qurl):
        print('actual URL:', qurl)
        ans = super().loadResource(rtype, qurl)
        if not ans:
            raise Exception(f'Failed to load: {qurl}')
        return ans
app = QApplication([])
d = D()
url = QUrl.fromLocalFile(os.path.abspath(__file__)).toString()
d.setHtml(f'<p><img src="{url}">')
print('expected url:', url)
img = QImage(100, 100, QImage.Format.Format_ARGB32_Premultiplied)
p = QPainter(img)
d.drawContents(p)
del d
del app 

Running this script produces no errors on Unic like OSes. On Windows it fails with the Exception Failed to load URL, because QTextDocument::loadResource() fails for the mangled URL passed to it.

This bug breaks using images with file:// URLs in all widgets that use QTextDocument which include, QTextEdit, QToolTip, QTextBrowser, etc. etc. It is not present Qt 5.