Push the blocks as far left as they go, then as far right; wherever a block lands both times, it is filled.
8before
8after
When to look for it
On the longest clues first, and again after a line gets tighter. It is the only rule that gives you anything on a completely empty grid.
Why it holds
Slide the run to the far left, then to the far right. Both are legal placements, so any square covered by both is covered wherever the run truly sits. Nothing about the rest of the line matters.
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.
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