on Windows 10, I recently switched the default app for viewing pdf files from Microsoft Edge to another chromium derivate. What I was lacking is the following:
The Table of Contents sidebar is closing itself when the focus is lost and there is no fix/pin option available
If I hit the "-"and "+" buttons the zoom is determined by the Browser settings, so in order to check what the zoom of the webpage/pdf file is, I have to hit the three dotted pull down menu and see what the actual number is. edit: I would expect the zoom percentage to be shown right next to the switches.
Maybe you could enhance this feature with the suggestions above.