Mind that proper GTK overlay scrollbars aren't transparent when hovered over. They're normal-width and largely (but not completely) opaque when hovered, thin and transparent while scrolling in other ways, and invisible otherwise. The thin scroll slider also has a dark fill and a light stroke, so stays visible regardless of the backdrop color.
Bug 1147847 Comment 18 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Mind that proper GTK overlay scrollbars aren't transparent when hovered over. They're normal-width and largely (but not completely) opaque when hovered, thin and transparent while scrolling in other ways, and invisible otherwise. The thin scroll slider also has a dark fill and a light stroke, so stays visible regardless of the backdrop color. Correction: The scrollbars are thin and transparent when scrolling in other ways *or* the mouse moves inside the viewport.