diff --git a/frontend/js/pane-manager.js b/frontend/js/pane-manager.js
index 8c6d052..c152de0 100644
--- a/frontend/js/pane-manager.js
+++ b/frontend/js/pane-manager.js
@@ -613,17 +613,7 @@ const PaneManager = {
 
   setActivePane(paneId) {
     if (this.activePaneId === paneId) return;
-    // Hide previous active pane content (lazy rendering)
-    if (this.panes[this.activePaneId] && this.panes[this.activePaneId].element) {
-      const oldContent = this.panes[this.activePaneId].element.querySelector('.content-area');
-      if (oldContent) oldContent.style.display = 'none';
-    }
     this.activePaneId = paneId;
-    // Show new active pane content
-    if (this.panes[paneId] && this.panes[paneId].element) {
-      const newContent = this.panes[paneId].element.querySelector('.content-area');
-      if (newContent) newContent.style.display = '';
-    }
     this._highlightActive();
     // Sync global state from this pane's active tab
     const tm = this.tabManagers[paneId];
diff --git a/tests/test_pane_manager.py b/tests/test_pane_manager.py
index 2e96249..a21a707 100644
--- a/tests/test_pane_manager.py
+++ b/tests/test_pane_manager.py
@@ -176,14 +176,17 @@ class TestPaneManagerDOMFlow:
         content = path.read_text()
         assert "id === 0" in content or "i === 0" in content
 
-    def test_lazy_rendering_hides_inactive(self):
-        """setActivePane hides old pane content with display:none."""
+    def test_panes_remain_visible_when_inactive(self):
+        """Split view keeps all panes visible — no display:none hiding.
+           Lazy rendering optimization removed because it broke split visibility."""
         path = PROJECT_ROOT / "frontend" / "js" / "pane-manager.js"
         content = path.read_text()
         active_section = content.split("setActivePane(paneId)")[1].split("  },")[0] \
             if "setActivePane(paneId)" in content else ""
-        assert "display" in active_section.lower(), \
-            "setActivePane must toggle display for lazy rendering"
+        # setActivePane should NOT hide inactive panes (they must stay visible)
+        assert "display" not in active_section.lower() or \
+               "display:" not in active_section.lower(), \
+            "setActivePane must NOT hide inactive panes — split view requires both visible"
 
     def test_collapse_restores_original_dom(self):
         """_collapseToSingle restores #tab-bar and #content-area IDs."""