It would be great if there was a way to force dolphin to open new instances of itself in a new tab, rather than new window, if it's launched already. This functionality seems to sparsely exist (open new folders in tabs), but now when launching dolphin itself it always goes to new window.
Folders opened from other apps' "Show containing folder" or "Open folder" action already open new tabs by default. Opening new instances, by contrast, is something the user always does deliberately. So instead of opening a new instance, why don't you open a new tab in the current instance if that's what you want?
it doesn't always open new tabs, for example in cases when usb is inserted and you select "open with file manager", a new instance is opened regardless. I suppose the biggest pro is that it would be a tad harder to clutter yourself up in cases where sometimes you use panel to open dolphin, sometimes ctrl t.
> for example in cases when usb is inserted and you select "open with file > manager", a new instance is opened regardless. That's a bug. :) In general when there are bugs, we'd prefer to fix bugs rather than provide new options to work around them. FWIW this works for me. If I have Dolphin open and I plug in a flash drive and hit "Open in file manager" in the popup, it opens a new tab in Dolphin, just like you want. The device notifier applet was partially re-done in Plasma 5.20 which is imminently about to be released, so it's quite possible that this was fixed there. I'll close this for now, but please to re-open it if you're still encountering that bug once you upgrade to Plasma 5.20.