When using this viewer part, a new tab (requested by Ctrl-LMB or MMB over a link, when the "Open links in new tab..." option is on) seems to be treated as a popup window and puts up a message box "This site is requesting to open a popup window to...". This is reasonable when a site is actually opening a popup, but for a user requested new tab it should not do this. The popup menu "Open in New Tab" action does not prompt and correctly just opens a new tab.
Neither KHTML nor the WebKit part put up this prompt, they just open a new tab as requested.
The fix to NewWindowPage::acceptNavigationRequest() checks the type of navigation request (passed up from QWebEnginePage) and does not prompt for user requests for a new tab.
The submitted diff is not reindented, so as not to confuse the code change with whitespace changes. I will indent the block in accordance with coding style before committing.