1. Load an image
2. Create a new segmentation image
3. Select the "Paint" tool and set a bigger brush size
4. Place the cursor on one of the 2D slice
5. Hold CTRL and move the mouse
6. Notice that the cursor leaves a trace
7. Release CTRL and move the mouse again
8. Notice that the cursor leaves another trace
9. Left click once and notice that the traces now become real segmentation

If you move the mouse to another of the 2D view before 9), the traces disappear.

I can reproduce the bug with the master commit 18c52c4cc9c63146e6c05f5fb825e79d07e8a3c3