I have already provided a valid test project for this, all that is needed is to use the latest version of the EO WebBrowser library to reproduce the issue. I have verified this is affecting 19.0.69 and 19.0.56, but does not affect versions from the 2018 series.

In the test project provided, after launching the project you can modify the url from the default to another url, such as https://www.essentialobjects.com/ then press tab.

In the test project, this moves the focus from the TextBox to the WebBrowser, however the TextBox never invokes the LostFocus event in this case.

If you use any version prior to 2019, this works without exception.

It should be noted that changing focus from the TextBox to another Focusable WPF control still invokes LostFocus, so this is limited to switching focus into the browser.