Avoid divide-by-zero in heading nav computation

This particular value can go to zero when the document height and the
window height are exactly the same value. This causes a NaN which causes
the "current" heading nav bug to not update properly. This clamps the
value to 1 to avoid that.

Author: ehuss

crates/mdbook-html/front-end/templates/toc.js.hbs

@@ -156,7 +156,7 @@ let mdbookThresholdDebug = false;
         // proportional to the difference in size.
         if (documentHeight < windowHeight * 2) {
             const maxPixelsBelow = documentHeight - windowHeight;
-            const t = 1 - pixelsBelow / maxPixelsBelow;
+            const t = 1 - pixelsBelow / Math.max(1, maxPixelsBelow);
             const clamp = Math.max(0, Math.min(1, t));
             bottomAdd *= clamp;
         }