fix: page down/up now scrolls by viewport height, not line count

The previous implementation counted logical lines in the viewport,
which failed when long wrapped lines consumed the entire viewport.
Now scrolls by actual pixel height for correct behavior.

Fixes #4562

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

src/static/js/ace2_inner.ts

@@ -2879,51 +2879,47 @@ function Ace2Inner(editorInfo, cssManagers) {
           // This is required, browsers will try to do normal default behavior on
           // page up / down and the default behavior SUCKS
           evt.preventDefault();
-          const oldVisibleLineRange = scroll.getVisibleLineRange(rep);
-          let topOffset = rep.selStart[0] - oldVisibleLineRange[0];
-          if (topOffset < 0) {
-            topOffset = 0;
-          }
 
           const isPageDown = evt.which === 34;
           const isPageUp = evt.which === 33;
 
-          scheduler.setTimeout(() => {
-            // the visible lines IE 1,10
-            const newVisibleLineRange = scroll.getVisibleLineRange(rep);
-            // total count of lines in pad IE 10
-            const linesCount = rep.lines.length();
-            // How many lines are in the viewport right now?
-            const numberOfLinesInViewport = newVisibleLineRange[1] - newVisibleLineRange[0];
-
-            if (isPageUp && padShortcutEnabled.pageUp) {
-              rep.selStart[0] -= numberOfLinesInViewport;
-              rep.selEnd[0] -= numberOfLinesInViewport;
+          if ((isPageDown && padShortcutEnabled.pageDown) ||
+              (isPageUp && padShortcutEnabled.pageUp)) {
+            // Scroll by actual viewport height in pixels, not by line count.
+            // This fixes the case where very long wrapped lines consume the
+            // entire viewport, making line-count-based scrolling useless.
+            const viewportHeight = outerWin.document.documentElement.clientHeight;
+            // Keep a small overlap so the user doesn't lose context
+            const scrollAmount = viewportHeight - 40;
+            const currentScrollY = scroll.getScrollY();
+
+            if (isPageDown) {
+              scroll.setScrollY(currentScrollY + scrollAmount);
+            } else {
+              scroll.setScrollY(Math.max(0, currentScrollY - scrollAmount));
             }
 
-            if (isPageDown && padShortcutEnabled.pageDown) {
-              rep.selStart[0] += numberOfLinesInViewport;
-              rep.selEnd[0] += numberOfLinesInViewport;
-            }
+            // Move cursor into the new visible area
+            scheduler.setTimeout(() => {
+              const linesCount = rep.lines.length();
+              const newVisibleRange = scroll.getVisibleLineRange(rep);
 
-            // clamp to valid line range
-            rep.selStart[0] = Math.max(0, Math.min(rep.selStart[0], linesCount - 1));
-            rep.selEnd[0] = Math.max(0, Math.min(rep.selEnd[0], linesCount - 1));
-            updateBrowserSelectionFromRep();
-            // get the current caret selection, can't use rep. here because that only gives
-            // us the start position not the current
-            const myselection = targetDoc.getSelection();
-            // get the carets selection offset in px IE 214
-            let caretOffsetTop = myselection.focusNode.parentNode.offsetTop ||
-                myselection.focusNode.offsetTop;
-
-            // sometimes the first selection is -1 which causes problems
-            // (Especially with ep_page_view)
-            // so use focusNode.offsetTop value.
-            if (caretOffsetTop === -1) caretOffsetTop = myselection.focusNode.offsetTop;
-            // set the scrollY offset of the viewport on the document
-            scroll.setScrollY(caretOffsetTop);
-          }, 200);
+              if (isPageDown) {
+                // Place cursor at the first line of the new viewport
+                rep.selStart[0] = newVisibleRange[0];
+                rep.selEnd[0] = newVisibleRange[0];
+              } else {
+                // Place cursor at the last line of the new viewport
+                rep.selEnd[0] = newVisibleRange[1];
+                rep.selStart[0] = newVisibleRange[1];
+              }
+
+              // clamp to valid line range
+              rep.selStart[0] = Math.max(0, Math.min(rep.selStart[0], linesCount - 1));
+              rep.selEnd[0] = Math.max(0, Math.min(rep.selEnd[0], linesCount - 1));
+              updateBrowserSelectionFromRep();
+            }, 0);
+          }
         }
       }