Moveable tabs, like in Firefox, would be very helpful to keep related pdfs grouped together. But only if Sumatra would remain the light and fast program it currently is.
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Tabs do move, just drag to right or left
The ability to move between windows is a long time open request https://github.com/sumatrapdfreader/sumatrapdf/issues/18
Combining groups of files into one tab is possible using in pre-release myfiles.vbkm feature , however it is still fairly experimental (page numbers are not what may be expected)
Blimey, I haven’t been paying attention. Sorry & thanks!