If the cursor lies upon the tab bar, turning mousewheel should change the active tab

Check your updates & origin of your packages/sources.
tab bar (standard one and static notebook) are scrollable and switch the active tab.

as of today the last published package is marlin 0.1-0~841~precise1 from ppa:marlin-devs/marlin-daily
If you re using random marlin's version from random ppa i can't garantee it's behavior.

Appears to be a problem with GTK 3.5-git; it's not working on nautilus anymore either. My bad.

(Wrote the report with r833; couldn't retry with r841 — compiled/built myself via an Arch Linux script, no ppa — since that one instantly goes to 100% CPU.)

Tried with 3.4.3; it's not working with that version either, also not in Nautilus.

However, the 100% CPU bug only occurs with 3.5.4. I've tested it with various recent commits reverted (since it worked until a few days ago), but no hit yet. Maybe it's in a build of GTK3 itself.

#3: s/build/dependency

Still no clue what exactly causes it, but at least I've been able to notice that it only occurs with certain GTK3 themes, such as Adwaita. No problems with eg. the default one or HighContrast[Inverse].

Removing other theme settings (such as prefer-dark) or installing an old version of gnome-themes-standard did nothing.