Only the left half of the resize handle in Live Data can be used

Steps to reproduce:

1. Open any Live Data, like the Wiki Manager.
2. With the mouse, move slowly from left to right over the area between two columns.

Expected result:

The resize handle stays visible while the mouse is on top of it.

Actual result:

The resize handle disappears even before the mouse reaches its center.

I've attached a screen cast showing this in 15.10.16 (unaffected) and 17.3.0 (affected).

This is because the resize handle is actually behind the next table header, its right half is visible but cannot be interacted with. This resize handle was introduced in XWIKI-21009. Before that, the resize handle was smaller and fully interactive, so this could even be considered a regression. This also impacts accessibility as it makes the area that needs to be hit with the mouse much smaller.