Improve the heading nav debug

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

@@ -78,10 +78,6 @@ window.customElements.define('mdbook-sidebar-scrollbox', MDBookSidebarScrollbox)
 // ---------------------------------------------------------------------------
 // Support for dynamically adding headers to the sidebar.
 
-// This is a debugging tool for the threshold which you can enable in the console.
-// eslint-disable-next-line prefer-const
-let mdbookThresholdDebug = false;
-
 (function() {
     // This is used to detect which direction the page has scrolled since the
     // last scroll event.
@@ -108,47 +104,27 @@ let mdbookThresholdDebug = false;
     // I'm not sure why eslint seems to have a false positive here.
     // eslint-disable-next-line prefer-const
     let headerToggles = [];
-
-    function drawDebugLine() {
-        if (!document.body) {
-            return;
-        }
-        const id = 'mdbook-threshold-debug-line';
-        const existingLine = document.getElementById(id);
-        if (existingLine) {
-            existingLine.remove();
-        }
-        const line = document.createElement('div');
-        line.id = id;
-        line.style.cssText = `
-            position: fixed;
-            top: ${threshold}px;
-            left: 0;
-            width: 100vw;
-            height: 2px;
-            background-color: red;
-            z-index: 9999;
-            pointer-events: none;
-        `;
-        document.body.appendChild(line);
-    }
+    // This is a debugging tool for the threshold which you can enable in the console.
+    let thresholdDebug = false;
 
     // Updates the threshold based on the scroll position.
     function updateThreshold() {
         const scrollTop = window.pageYOffset || document.documentElement.scrollTop;
         const windowHeight = window.innerHeight;
         const documentHeight = document.documentElement.scrollHeight;
+
         // The number of pixels below the viewport, at most documentHeight.
         // This is used to push the threshold down to the bottom of the page
         // as the user scrolls towards the bottom.
         const pixelsBelow = Math.max(0, documentHeight - (scrollTop + windowHeight));
-        // The number of pixels above the viewport, at most defaultDownThreshold.
+        // The number of pixels above the viewport, at least defaultDownThreshold.
         // Similar to pixelsBelow, this is used to push the threshold back towards
         // the top when reaching the top of the page.
         const pixelsAbove = Math.max(0, defaultDownThreshold - scrollTop);
         // How much the threshold should be offset once it gets close to the
         // bottom of the page.
-        let bottomAdd = Math.max(0, windowHeight - pixelsBelow - defaultDownThreshold);
+        const bottomAdd = Math.max(0, windowHeight - pixelsBelow - defaultDownThreshold);
+        let adjustedBottomAdd = bottomAdd;
 
         // Adjusts bottomAdd for a small document. The calculation above
         // assumes the document is at least twice the windowheight in size. If
@@ -158,7 +134,7 @@ let mdbookThresholdDebug = false;
             const maxPixelsBelow = documentHeight - windowHeight;
             const t = 1 - pixelsBelow / maxPixelsBelow;
             const clamp = Math.max(0, Math.min(1, t));
-            bottomAdd *= clamp;
+            adjustedBottomAdd *= clamp;
         }
 
         let scrollingDown = true;
@@ -169,10 +145,10 @@ let mdbookThresholdDebug = false;
         if (scrollingDown) {
             // When scrolling down, move the threshold up towards the default
             // downwards threshold position. If near the bottom of the page,
-            // bottomAdd will offset the threshold towards the bottom of the
-            // page.
+            // adjustedBottomAdd will offset the threshold towards the bottom
+            // of the page.
             const amountScrolledDown = scrollTop - lastKnownScrollPosition;
-            const adjustedDefault = defaultDownThreshold + bottomAdd;
+            const adjustedDefault = defaultDownThreshold + adjustedBottomAdd;
             threshold = Math.max(adjustedDefault, threshold - amountScrolledDown);
         } else {
             // When scrolling up, move the threshold down towards the default
@@ -181,12 +157,81 @@ let mdbookThresholdDebug = false;
             // belongs.
             const amountScrolledUp = lastKnownScrollPosition - scrollTop;
             const adjustedDefault = defaultUpThreshold - pixelsAbove
-                + Math.max(0, bottomAdd - defaultDownThreshold);
+                + Math.max(0, adjustedBottomAdd - defaultDownThreshold);
             threshold = Math.min(adjustedDefault, threshold + amountScrolledUp);
         }
+
+        if (documentHeight <= windowHeight) {
+            threshold = 0;
+        }
+
+        if (thresholdDebug) {
+            const id = 'mdbook-threshold-debug-data';
+            let data = document.getElementById(id);
+            if (data === null) {
+                data = document.createElement('div');
+                data.id = id;
+                data.style.cssText = `
+                    position: fixed;
+                    top: 50px;
+                    right: 10px;
+                    background-color: 0xeeeeee;
+                    z-index: 9999;
+                    pointer-events: none;
+                `;
+                document.body.appendChild(data);
+            }
+            data.innerHTML = `
+                <table>
+                  <tr><td>documentHeight</td><td>${documentHeight.toFixed(1)}</td></tr>
+                  <tr><td>windowHeight</td><td>${windowHeight.toFixed(1)}</td></tr>
+                  <tr><td>scrollTop</td><td>${scrollTop.toFixed(1)}</td></tr>
+                  <tr><td>pixelsAbove</td><td>${pixelsAbove.toFixed(1)}</td></tr>
+                  <tr><td>pixelsBelow</td><td>${pixelsBelow.toFixed(1)}</td></tr>
+                  <tr><td>bottomAdd</td><td>${bottomAdd.toFixed(1)}</td></tr>
+                  <tr><td>adjustedBottomAdd</td><td>${adjustedBottomAdd.toFixed(1)}</td></tr>
+                  <tr><td>scrollingDown</td><td>${scrollingDown}</td></tr>
+                  <tr><td>threshold</td><td>${threshold.toFixed(1)}</td></tr>
+                </table>
+            `;
+            drawDebugLine();
+        }
+
         lastKnownScrollPosition = scrollTop;
     }
 
+    function drawDebugLine() {
+        if (!document.body) {
+            return;
+        }
+        const id = 'mdbook-threshold-debug-line';
+        const existingLine = document.getElementById(id);
+        if (existingLine) {
+            existingLine.remove();
+        }
+        const line = document.createElement('div');
+        line.id = id;
+        line.style.cssText = `
+            position: fixed;
+            top: ${threshold}px;
+            left: 0;
+            width: 100vw;
+            height: 2px;
+            background-color: red;
+            z-index: 9999;
+            pointer-events: none;
+        `;
+        document.body.appendChild(line);
+    }
+
+    function mdbookEnableThresholdDebug() {
+        thresholdDebug = true;
+        updateThreshold();
+        drawDebugLine();
+    }
+
+    window.mdbookEnableThresholdDebug = mdbookEnableThresholdDebug;
+
     // Updates which headers in the sidebar should be expanded. If the current
     // header is inside a collapsed group, then it, and all its parents should
     // be expanded.
@@ -210,9 +255,6 @@ let mdbookThresholdDebug = false;
     // This is done with a virtual Y threshold, where headers at or below
     // that line will be considered the current one.
     function updateCurrentHeader() {
-        if (mdbookThresholdDebug) {
-            drawDebugLine();
-        }
         if (!headers || !headers.length) {
             return;
         }