Oh, so that's a separator? I see, now. When Apply Colors is off, the separator always exists between background tabs. However when Apply Colors is on, the separator becomes unpredictable. Sometimes it appears on mouseover, sometimes not. Sometimes it's left behind, sometimes it's gone.
Onto your question. Now that I see it, the background tabs in Explorer seem to be unpainted, too, except on mouseover, and I see the same style of vertical separator there. I don't know how I missed that. So the XY separator should exist. The separator seems semi-transparent in Explorer. For example, if the Explorer titlebar is full black, the separator between background tabs samples to #151515 (RGB 21,21,21, or 8% lum). If the Explorer titlebar background is a dark grey #0a0a0a, its separator lightens in relation to it to #1e1e1e (RGB 30,30,30, or 12% lum).
TLDR; Background tabs in rounded mode look better when they aren't shown, much more Explorer-like, but the separator is too light.
So.... a custom separator colour tweak?
With build 1200 onward, I have some confusion on the role and difference between "Apply Colors" checkbox versus "Preserve Custom Colors".
Alone, Preserve Custom Colors will make foreground tabs keep any specialized per-tab custom *background* colour, but then the Breadcrumb text colour overrides. Meanwhile, Apply Colours on its own will prevent Breadcrumb text from overriding Active Tab/Other Tab text colour (great!), but it simultaneously does something unexpected (unwanted?) -- it forces per-tab custom colours which I thought was the role of "Preserve Custom Colors" only. If I check both boxes, there is no visible difference than just "Apply Colours". I could just remove all custom per-tab colour definitions if I want to make them uniform. But the checkboxes exist!
So Apply Colors and Preserve Custom Colors appear to be swapped? Or Apply Colours *shouldn't* be also applying Custom Colors unless both are checked?