Merge branch 'paste-buffer-large'

Buffers large pastes outside the textarea to prevent the
multi-second UI freeze that happens when megabytes of text get
committed to a <textarea> in one shot. Threshold 256 KB; first 50
lines + a banner render into the textarea, full content uploads
on Save.

web/public/js/start.js

@@ -7,8 +7,27 @@ const fileSelectButton = document.getElementById('paste-select-file');
 const pasteClipboardButton = document.getElementById('paste-clipboard');
 const pasteError = document.getElementById('paste-error');
 
+/*
+ * Large-paste buffering. <textarea> rendering is the bottleneck once content
+ * crosses a few hundred KB; the browser locks up while it lays out millions
+ * of characters. For pastes / file loads above the threshold, we hold the
+ * full content in `bufferedContent` and only render a small preview into the
+ * textarea. The save path uses `bufferedContent` when set.
+ */
+const PASTE_BUFFER_THRESHOLD_BYTES = 256 * 1024; // 256 KB
+const PASTE_PREVIEW_LINES = 50;
+let bufferedContent = null;
+
 pasteArea.focus();
-pasteArea.addEventListener('input', reevaluateContentStatus);
+pasteArea.addEventListener('input', () => {
+    // User-typed input invalidates the buffer (the textarea is now the
+    // source of truth). The buffer is only set programmatically, so a
+    // user-driven `input` event always means edit-from-preview.
+    if (bufferedContent !== null) {
+        clearBuffer();
+    }
+    reevaluateContentStatus();
+});
 pasteArea.addEventListener('paste', handlePasteEvent);
 pasteSaveButtons.forEach(button => button.addEventListener('click', sendLog));
 fileSelectButton.addEventListener('click', selectLogFile);
@@ -39,7 +58,7 @@ async function sendLog() {
     pasteSaveButtons.forEach(button => button.classList.add("btn-working"));
 
     try {
-        let log = pasteArea.value;
+        let log = bufferedContent ?? pasteArea.value;
         log = applyFilters(log);
 
         const bodyData = {
@@ -149,13 +168,56 @@ async function pasteFromClipboard() {
             showError("Clipboard is empty.");
             return;
         }
-        pasteArea.value = content;
-        reevaluateContentStatus();
+        loadContent(content);
     } catch (err) {
         showError("Clipboard is empty or not accessible.");
     }
 }
 
+/*
+ * Single entry point for "place this string in the editor."
+ * Routes large content into the buffer + preview; small content goes into
+ * the textarea normally so the user can keep editing it.
+ */
+function loadContent(text) {
+    if (text.length > PASTE_BUFFER_THRESHOLD_BYTES) {
+        loadIntoBuffer(text);
+    } else {
+        clearBuffer();
+        pasteArea.value = text;
+        reevaluateContentStatus();
+    }
+}
+
+function loadIntoBuffer(text) {
+    bufferedContent = text;
+    const lines = text.split('\n');
+    const sizeMb = (text.length / 1024 / 1024).toFixed(2);
+    if (lines.length <= PASTE_PREVIEW_LINES) {
+        pasteArea.value = text;
+    } else {
+        const preview = lines.slice(0, PASTE_PREVIEW_LINES).join('\n');
+        const remaining = lines.length - PASTE_PREVIEW_LINES;
+        pasteArea.value =
+            `[Large paste buffered: ${lines.length.toLocaleString()} lines, ${sizeMb} MB.\n` +
+            ` Full content uploads on Save. Edit this textarea to clear the buffer.]\n` +
+            `\n` +
+            `--- preview: first ${PASTE_PREVIEW_LINES} of ${lines.length.toLocaleString()} lines ---\n` +
+            preview +
+            `\n--- ${remaining.toLocaleString()} more lines hidden ---\n`;
+    }
+    pasteArea.readOnly = true;
+    reevaluateContentStatus();
+}
+
+function clearBuffer() {
+    if (bufferedContent === null) {
+        return;
+    }
+    bufferedContent = null;
+    pasteArea.readOnly = false;
+}
+
 function reevaluateContentStatus() {
     clearError();
     if (pasteArea.value.length > 0) {
@@ -184,6 +246,14 @@ async function handlePasteEvent(e) {
     if (e.clipboardData?.files?.length > 0) {
         e.preventDefault();
         await loadFileContents(e.clipboardData.files[0]);
+        return;
+    }
+    // Intercept large text pastes before the browser commits them to the
+    // textarea — assigning multi-MB strings to a <textarea> freezes the page.
+    const text = e.clipboardData?.getData('text');
+    if (text && text.length > PASTE_BUFFER_THRESHOLD_BYTES) {
+        e.preventDefault();
+        loadIntoBuffer(text);
     }
 }
 
@@ -220,8 +290,7 @@ async function loadFileContents(file) {
         return;
     }
 
-    pasteArea.value = new TextDecoder().decode(content);
-    reevaluateContentStatus();
+    loadContent(new TextDecoder().decode(content));
 }
 
 function selectLogFile() {
@@ -355,8 +424,7 @@ async function handleDropEvent(e) {
     let files = e.dataTransfer.files;
     if (files.length !== 1) {
         if (Array.from(e.dataTransfer.types).includes('text/plain')) {
-            pasteArea.value = e.dataTransfer.getData('text/plain');
-            reevaluateContentStatus();
+            loadContent(e.dataTransfer.getData('text/plain'));
         }
         return;
     }