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@Boldline The gray marker of the eraser tool shows the area that will be erased after the mouse is released.
The eraser tool uses the same curve smoothing as the pencil tool, and if this smoothness value is set to large, the whole erased paint may change slightly as the mouse is moved. This is indicated by the gray marker.
Erasing occurs only after the mouse is released, and it is all done as a vector operation.
And only the selected objects are affected (that is, the object that were selected when the erase tool was activated).
If no objects were selected, an object is selected (for erasure) when the mouse is pressed.
Other objects are not affected by the erase tool.