Challenge 2: Paint Only Appearing in a Small Rectangular Area — Solution Walkthrough

The most likely cause:

A leftover selection is still active from an earlier step — most
likely a rectangular Marquee/Rectangle Select selection made previously
and never deselected. Per this chapter's own core rule, edits only
affect pixels within the active selection; since a small rectangular
area is exactly where the paint is appearing, that rectangular shape
is a strong sign that a rectangular selection is quietly restricting
where the brush tool can actually apply color.

Why this matches this chapter's own warning box directly:

This is precisely the scenario the warning box describes — a tool
appearing not to work (paint only showing in part of the canvas) when
the real cause is a forgotten, still-active selection from a previous
step, not a malfunctioning brush tool.

The fix:

Check Select > Deselect (or GIMP's equivalent) to confirm whether a
selection is still active, and deselect it if so. Once no selection
restricts the canvas, the brush tool should paint across the entire
image normally.

WHY THIS WORKS AS AN ANSWER
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This exercise directly applies this chapter's own warning box to a
concrete symptom (a rectangular painted area), correctly tracing the
shape of the affected area back to a specific likely selection type
still being active.