yeah so ... Qt Broke it. Was not broken in past version (not with the version I have) (5.9.4)
Note that what is broken is the rendering (basically Qt having issues with transparent widgets), not the functionality. (the ability to resize splitter in a region that is larger than the actual splitter width).

Disabling it will:

• fix the rendering issue in the cases where it is broken
• make resizing using splitter nearly impossible.

Bottomline: I don't think this patch is enough.

What you want (I think) is that when splitterHandle is disabled, splitters get larger. (at the minimum 3 pixels, or even more).
This requires some extra changes in breezeStyle.

I'm running with the splitter proxy disabled in the configuration dialog and i don't have any problem moving splitters.

In D10830#217600, @aacid wrote:

Hmmmmmmm

I just realized this is weird, so the splitter in the kate sidebar is exactly 1 pixel wide so as you mentioned it can be a bit hard to grab (specially if using a touchpad) but the splitters in KMail and Konversation are several pixels wide (i'd say even 4 or 5).

Do you know why this may be?

This is not in breeze directly, as far as I know.
Could it be the difference between QSplitter and QDockWidget ?
In any case, no clue.
I'll investigate.
The other possibility of course would be to "fix" the splitter proxy. Ideally one would get rid of the transparent widget and just use proper event filters. But this requires time and testing.