KDE no longer automatically switches desktops

**Background** I have been using Linux Mint KDE v17.3 (Plasma v4.13.2) for a number of years. I keep Firefox open in one virtual desktop and Thunderbird open in another. If I receive an email in Thunderbird containing a link and click on the link, the link opens in Firefox and Plasma automatically switches desktops to view it. **Symptom** I just installed Kubuntu 18.04 with Plasma 5.12.7. It behaves differently, and it looks like recent Plasma versions on other distros are similar. - With the same virtual desktop setup, clicking a link in Thunderbird opens the link in Firefox but does not switch desktops. The Task Manager icon changes color to indicate the activity on the other desktop, but you need to manually switch desktops to view it. - If Firefox is not already open, clicking the link in Thunderbird will launch Firefox (or a different browser if selected), in the same desktop as Thunderbird and switch focus to the browser. Similarly, if the browser is already open in the same desktop, focus will switch to it. The only difference in behavior is that Plasma will no longer switch desktops if needed to make the browser visible. **Additional testing** - I've tested the behavior on two computers and a number of other distros using recent versions of Plasma (Mint 19 with the Kubuntu's Plasma 5.12.7 desktop, Neptune [KDE 5.12.7 on Debian], openSUSE v42.3 w/Plasma 5.8.7, Manjaro w/Plasma 5.15.0). They all behave the same way as Kubuntu. I also tested it in a live session of KDE Neon v5.12.4 and that behaved the same way. - I've also tested this with sources of links other than Thunderbird and with other browsers and the behavior is the same. - The style of desktop arrangement (grid, cube, etc.), and type of switching animation have no effect on the symptom. So the source appears to be recent versions of Plasma. It isn't clear whether this is new intended behavior for Plasma or a bug. I couldn't find any references to this behavior in online searches. Is there a way to restore the previous behavior of automatically switching desktops?