Tested in KDE Neon.
Here's Dolphin's main window with multiple vertical panels with the current default setting:
{F3922996}
Notice how content in each panel is cut off at the splitter without any kind of visual cue that there's one there. It looks like a glitch, and an average wouldn't know that it's resizable unless they happened to mouse over it.
Here's Dolphin's main window with multiple vertical panels with the patch:
{F3922997}
Now you can clearly see that there's a splitter separating the content views, and it looks resizable.