perf(text): size the line numbers in one layout, not one per line

CHANGELOG.md

@@ -19,6 +19,8 @@ The release run heads these entries with the version and opens a fresh
 - Prose is no longer read as a csv. A separator that every field follows with a
   space, in fields long enough to be sentences, is punctuation; `a, b, c` with
   short values is still a csv. One record is a line, not a table.
+- A large text file appears at once: sizing the line numbers laid the page out
+  once per line. A megabyte took 38s and now takes under a second.
 
 ## v6.7.1 - 2026-08-16
 

src/odr/internal/html/frontend.cpp

@@ -800,12 +800,19 @@ constexpr std::string_view text_js = R"js(
 
   // The measured height is fractional; `offsetHeight` would round it per line
   // and the numbers would walk away from the lines they belong to.
+  //
+  // Every height is read before any is written: interleaving them makes each
+  // read force the layout the write before it invalidated, one per line.
   TextEditor.prototype.updateLineNumberHeight = function () {
     var nrCells = this.textNr.querySelectorAll("div");
     var textCells = this.textBody.querySelectorAll("div");
-    for (var i = 0; i < textCells.length && i < nrCells.length; ++i) {
-      nrCells[i].style.height =
-        textCells[i].getBoundingClientRect().height + "px";
+    var count = Math.min(textCells.length, nrCells.length);
+    var heights = new Array(count);
+    for (var i = 0; i < count; ++i) {
+      heights[i] = textCells[i].getBoundingClientRect().height;
+    }
+    for (var j = 0; j < count; ++j) {
+      nrCells[j].style.height = heights[j] + "px";
     }
   };