line multi authors

static/css/styles.css

@@ -4057,3 +4057,39 @@ dialog::backdrop {
     grid-template-columns: 1fr;
   }
 }
+
+/* Per-character authorship overlay. The textarea remains the editable surface. */
+.authorship-layer {
+  position: absolute;
+  z-index: 1;
+  inset: 0 0 0 48px;
+  overflow: hidden;
+  padding: 24px;
+  color: transparent;
+  font: inherit;
+  font-size: var(--editor-font-size, 14px);
+  line-height: inherit;
+  white-space: pre;
+  tab-size: 4;
+  pointer-events: none;
+}
+
+.authorship-fragment {
+  border-radius: 2px;
+  background: color-mix(in srgb, var(--owner) 18%, transparent);
+  box-shadow: inset 0 -2px color-mix(in srgb, var(--owner) 72%, transparent);
+  color: transparent;
+}
+
+.editor-shell textarea {
+  position: relative;
+  z-index: 3;
+  background: transparent;
+}
+
+.hide-editor-line-numbers .authorship-layer { left: 0; }
+
+@media (max-width: 720px) {
+  .authorship-layer { left: 42px; }
+  .hide-editor-line-numbers .authorship-layer { left: 0; }
+}