perf(logging): share immutable style snapshots between cells

Copy style state on SGR changes rather than for each printable character. Preserve historical styles, cleared screens and cursor overwrites without dropping log content.

electron/bridges/terminalLogSanitizer.cjs

@@ -292,6 +292,9 @@ class TerminalTextRenderer {
   }
 
   #applySgr(values) {
+    // Cells share style snapshots. Copy before SGR mutations so earlier output
+    // (including preserved cleared screens) keeps its original appearance.
+    this.style = { ...this.style };
     const codes = values.length > 0 ? values : [0];
 
     for (let i = 0; i < codes.length; i += 1) {
@@ -544,7 +547,7 @@ function createDefaultStyle() {
 function createCell(ch, style) {
   return {
     ch,
-    style: { ...style },
+    style,
   };
 }
 

electron/bridges/terminalLogSanitizer.test.cjs

@@ -330,3 +330,52 @@ test("erasing styled spaces updates the visible line timestamp", () => {
   renderer.feed("\r\x1b[2C\x1b[1K", 9000);
   assert.equal(renderer.toHtmlContent({ formatLine }), "[9000]    text");
 });
+
+test("cells share style snapshots without recoloring earlier output on SGR changes", () => {
+  const renderer = createTerminalTextRenderer();
+  renderer.feed("ab\x1b[31mcd\x1b[1mef\x1b[0mgh");
+  const cells = renderer.lines[0];
+  assert.equal(cells[0].style, cells[1].style);
+  assert.equal(cells[2].style, cells[3].style);
+  assert.notEqual(cells[1].style, cells[2].style);
+  assert.notEqual(cells[3].style, cells[4].style);
+  assert.equal(renderer.toString(), "abcdefgh");
+  assert.equal(renderer.toHtmlContent(), 'ab<span style="color: var(--ansi-1, #cd3131)">cd</span><span style="color: var(--ansi-1, #cd3131); font-weight: 700">ef</span>gh');
+});
+
+test("SGR attribute and color resets leave previous snapshots unchanged", () => {
+  const renderer = createTerminalTextRenderer();
+  renderer.feed("\x1b[1;3;4;7;38;5;196;48;2;1;2;3mA");
+  const snapshot = renderer.lines[0][0].style;
+  const original = { ...snapshot };
+  renderer.feed("\x1b[22;23;24;27;39;49mB\x1b[0;32;1mC\x1b[mD");
+  assert.deepEqual(snapshot, original);
+  assert.deepEqual(original, { fg: "#ff0000", bg: "#010203", bold: true, italic: true, underline: true, inverse: true });
+  assert.deepEqual(renderer.lines[0][1].style, { fg: null, bg: null, bold: false, italic: false, underline: false, inverse: false });
+  assert.equal(renderer.toString(), "ABCD");
+  assert.equal(renderer.toHtmlContent(), '<span style="color: #010203; background-color: #ff0000; font-weight: 700; font-style: italic; text-decoration: underline">A</span>B<span style="color: var(--ansi-2, #0dbc79); font-weight: 700">C</span>D');
+});
+
+test("cursor overwrites and erasure do not mutate adjacent shared styles", () => {
+  const renderer = createTerminalTextRenderer();
+  renderer.feed("\x1b[31mabcd\r\x1b[32mX\x1b[3G\x1b[0mY");
+  assert.equal(renderer.toString(), "XbYd");
+  assert.equal(renderer.toHtmlContent(), '<span style="color: var(--ansi-2, #0dbc79)">X</span><span style="color: var(--ansi-1, #cd3131)">b</span>Y<span style="color: var(--ansi-1, #cd3131)">d</span>');
+  renderer.feed("\x1b[1K");
+  assert.equal(renderer.toHtmlContent(), "");
+});
+
+test("pending cleared-screen clones share immutable styles but retain independent cells", () => {
+  const renderer = createTerminalTextRenderer();
+  renderer.feed("first\x1b[H\x1b[2J\x1b[31mold", 10);
+  const activeLine = renderer.lines.at(-1);
+  renderer.feed("\x1b[H\x1b[2J");
+  const savedLine = renderer.pendingClearedScreen.lines[0];
+  assert.notEqual(savedLine[0], activeLine[0]);
+  assert.equal(savedLine[0].style, activeLine[0].style);
+  assert.equal(savedLine.timestamp, 10);
+  renderer.feed("\x1b[32mnew", 20);
+  assert.equal(savedLine[0].style.fg, "var(--ansi-1, #cd3131)");
+  assert.equal(renderer.toString({ includePendingClearedScreen: true }), "first\nold\n\nnew");
+  assert.equal(renderer.toHtmlContent({ includePendingClearedScreen: true }), 'first\n<span style="color: var(--ansi-1, #cd3131)">old</span>\n\n<span style="color: var(--ansi-2, #0dbc79)">new</span>');
+});