Kubuntu 18.10 - How do I change this file picker?

Solution 1:

You want applications to use the KDE file selection dialog so that you can have the appearance matching that of Dolphin.

As you have seen, native applications such as Spectacle do conform, but others don't. This is probably because the non-conforming applications are gtk-based and use the GTK file selection dialog:  

Firefox uses the gtk file selection dialog by default

To get Firefox, which is a gtk application, to use the KDE file selection dialog, start it from a terminal like this:

GTK_USE_PORTAL=1 firefox

Now, you'll see  

Similarly, you can make gedit, and other gtk applications, use the KDE file selection dialog.  

Here's what you'll see with GTK_USE_PORTAL=1 gedit:  

If you're sure you want to consistently use the KDE file selection dialog, you can add export GTK_USE_PORTAL=1 to the end of ~/.profile. After you log out and log back in, all applications will use the KDE file selection dialog.

Note that the following two packages should be present in your system:

• xdg-desktop-portal-kde
• xdg-desktop-portal