Normal desktop mode, freshly opened main Steam window showing the library view (or something else locally-generated).
Select something which causes a web page to be displayed – doesn't matter what, so long as it replaces the library view. (The bug seems more likely to happen if you open the settings window and click on something suitable there.)
Click on the back button. It takes you back to the library.
Re-open the web page.
Press Alt-Left (or AltGr-Left).
Notice how it takes you back not to the library view (if that's what you had open), but a web browser view with the corresponding data: URL.
(Alt-Left & Alt-Right only work if the web view has the input focus.)
Notice how it takes you back not to the library view (if that's what you had open), but a web browser view with the corresponding
data:URL.(Alt-Left & Alt-Right only work if the web view has the input focus.)