This gap is shorter than the smallest block left, so nothing fits and all of it is empty.
2before
2after
When to look for it
After crossing squares off, when the line has broken into small gaps.
Why it holds
A gap bounded by crossed squares can only hold a block that fits inside it. If it is shorter than the smallest block still unplaced, nothing can go there and all of it is empty.
Try it
Mark every square this rule settles, then check.
Click a square once to fill it, twice to cross it off, three times to clear it.
2
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