To reproduce:
1. Open any project with ProBuilder
2. Open ProBuilder Debug Window (Tools>Probuidler>Debug> ProBuilder Debug Window)
3. Create any ProBuilder shape with 10+ shapes
4. In Game View, switch to Face Selection mode
5. While holding Shift, keep selecting faces
6. Observe ProBuilder Debug Window

Expected: values of Line Length and Vertex Offset are not changed
Actual: after selecting about 5 faces, Line Length and Vertex Offset will start to increase with each new selection

Reproduced in: 2018.1.2f1, 2018.2.0b6

Note1: only tested on releases with integrated ProBuilder due to older releases being obsolete
Note2: If values are set manually, the issue does not occur
Note3: slider values are shown to be increased, however, it does not affect the parameters themselves

---

The Debug window is deprecated in ProBuilder 4.0.0.