Comment 4 for bug 1647292

1.90 is the gtk3 release of Terminator, and *only* uses gtk3 theming. The gtk2 theme has zero effect on 1.90. It only affects the gtk2 Terminator (i.e. 0.98/1.0) release. So you not changing the gtk2 theme doesn't matter, because the gtk2 and gtk3 themes do not share anything. The best you can hope for is a theme that provides for both gtk2 and gtk3 and makes them as consistent as possible with each other given their respective restrictions. This is the kind of thing dsitros like Ubuntu do, with their Ambiance/Radiance themes. The way to test for certain is to find another gtk3 program that uses splitters, and to test it there too.

Simplest is to check gtk3-demo and select "Paned Widgets" on the left, then Run at the top. There is one of each orientation. See if the handles hover area is oversized there too. Although this is not definitive, as that would depend on the other program letting you modify splitter handle width on the fly too :-) Increasing/decreasing the separator size should be a global dynamic change during runtime, as soon as you close the Preferences window.