The polygon selection tool does not close the selection, there is a stray selection line going off the screen to the tools window on the left.
You did not double-click on screen or return to the starting point of the selection area to close the shape before they selected a NEW tool.
The stray line points directly to the new tool selection.
Reselect the polygon tool and complete the selection by either double clicking the mouse on the screen or returning to the starting point of the selection and clicking the mouse once.
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