[KevinH]

Does it help to force the use of the execCommand javascript like we used for Qt 5.9.5 instead:

See this snippet in PageEdit's MainWindow.cpp:

#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
void MainWindow::Bold()               { m_WebView->triggerPageAction(QWebEnginePage::ToggleBold);          }
void MainWindow::Italic()             { m_WebView->triggerPageAction(QWebEnginePage::ToggleItalic);        }
void MainWindow::Underline()          { m_WebView->triggerPageAction(QWebEnginePage::ToggleUnderline);     }
void MainWindow::Strikethrough()      { m_WebView->triggerPageAction(QWebEnginePage::ToggleStrikethrough); }
void MainWindow::AlignLeft()          { m_WebView->triggerPageAction(QWebEnginePage::AlignLeft);           }
void MainWindow::AlignCenter()        { m_WebView->triggerPageAction(QWebEnginePage::AlignCenter);         }
void MainWindow::AlignRight()         { m_WebView->triggerPageAction(QWebEnginePage::AlignRight);          }
void MainWindow::AlignJustify()       { m_WebView->triggerPageAction(QWebEnginePage::AlignJustified);      }
void MainWindow::DecreaseIndent()     { m_WebView->triggerPageAction(QWebEnginePage::Outdent);             }
void MainWindow::IncreaseIndent()     { m_WebView->triggerPageAction(QWebEnginePage::Indent);              }
void MainWindow::InsertBulletedList() { m_WebView->triggerPageAction(QWebEnginePage::InsertUnorderedList); }
void MainWindow::InsertNumberedList() { m_WebView->triggerPageAction(QWebEnginePage::InsertOrderedList);   }
#else
void MainWindow::Bold()               { m_WebView->ExecCommand("bold");                }
void MainWindow::Italic()             { m_WebView->ExecCommand("italic");              }
void MainWindow::Underline()          { m_WebView->ExecCommand("underline");           }
void MainWindow::Strikethrough()      { m_WebView->ExecCommand("strikeThrough");       }
void MainWindow::AlignLeft()          { m_WebView->ExecCommand("justifyLeft");         }
void MainWindow::AlignCenter()        { m_WebView->ExecCommand("justifyCenter");       }
void MainWindow::AlignRight()         { m_WebView->ExecCommand("justifyRight");        }
void MainWindow::AlignJustify()       { m_WebView->ExecCommand("justifyFull");         }
void MainWindow::DecreaseIndent()     { m_WebView->ExecCommand("outdent");             }
void MainWindow::IncreaseIndent()     { m_WebView->ExecCommand("indent");              }
void MainWindow::InsertBulletedList() { m_WebView->ExecCommand("insertUnorderedList"); }
void MainWindow::InsertNumberedList() { m_WebView->ExecCommand("insertOrderedList");   }
#endif