## Summary
**Fixed:** Invalid layout state where only left and right sidebars are visible (no main content area).
### Changes Made:
**1. `frontend/src/stores/useLayoutStore.ts`**
- Added `setMainPanelVisible` setter function to the type and implementation
- Added `useIsRightMainPanelVisible` derived selector hook that checks if Changes/Logs/Preview panel is visible
**2. `frontend/src/components/ui-new/containers/WorkspacesLayout.tsx`**
- Imported `useIsRightMainPanelVisible` from the layout store
- Added `setMainPanelVisible` to the destructured store values
- Replaced verbose `isChangesMode || isLogsMode || isPreviewMode` checks with the helper
- Added guard effect that ensures the left main panel (chat) is visible when the right main panel is hidden - this prevents the invalid state after page reload
The guard effect automatically opens the main panel whenever the layout would otherwise show only sidebars:
```typescript
// Ensure left main panel (chat) is visible when right main panel is hidden
// This prevents invalid state where only sidebars are visible after page reload
useEffect(() => {
if (!isMainPanelVisible && !isRightMainPanelVisible) {
setMainPanelVisible(true);
}
}, [isMainPanelVisible, isRightMainPanelVisible, setMainPanelVisible]);
```
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