Prevent collapsing all channels - ensure at least one remains expanded

Resolves issue where collapsing all channels would result in an empty window
with no way to recover. The last remaining expanded channel is now protected
from being collapsed.

Changes:
- Added logic in toggleChannelCollapse to check if collapsing would hide all channels
- If attempting to collapse the last expanded channel, the action is prevented
- Added disabled state to collapse button when it's the last expanded channel
- Button shows tooltip "Cannot collapse last channel" when disabled
- Added CSS for disabled button state (reduced opacity, not-allowed cursor)
- Expanding channels is always allowed regardless of state

This ensures users always have at least one visible channel to interact with
and prevents the unrecoverable empty window state.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MarkoVcode

benchmesh-serial-service/frontend/src/ui/workbench/EditorArea/MosaicInstrumentWindow.tsx

@@ -88,9 +88,19 @@ export const MosaicInstrumentWindow: React.FC<MosaicInstrumentWindowProps> = ({
   const toggleChannelCollapse = (channelPath: string) => {
     setCollapsedChannels(prev => {
       const newSet = new Set(prev);
+
+      // Calculate total number of channels across all classes
+      const totalChannels = classesToDisplay.reduce((sum: number, c: any) => sum + c.channels.length, 0);
+
       if (newSet.has(channelPath)) {
+        // Expanding - always allow
         newSet.delete(channelPath);
       } else {
+        // Collapsing - prevent if this is the last expanded channel
+        if (newSet.size >= totalChannels - 1) {
+          // Would collapse all channels - prevent this
+          return prev;
+        }
         newSet.add(channelPath);
       }
       return newSet;
@@ -118,6 +128,10 @@ export const MosaicInstrumentWindow: React.FC<MosaicInstrumentWindowProps> = ({
                     const channelColor = channelColors[channelNum] || '#808080'; // Gray fallback
                     const isCollapsed = collapsedChannels.has(channelPath);
 
+                    // Calculate if this is the last expanded channel
+                    const totalChannels = classesToDisplay.reduce((sum: number, cls: any) => sum + cls.channels.length, 0);
+                    const isLastExpanded = !isCollapsed && collapsedChannels.size >= totalChannels - 1;
+
                     return (
                       <div
                         key={channelPath}
@@ -136,7 +150,14 @@ export const MosaicInstrumentWindow: React.FC<MosaicInstrumentWindowProps> = ({
                             <button
                               className="channel-collapse-toggle"
                               onClick={() => toggleChannelCollapse(channelPath)}
-                              title={isCollapsed ? 'Expand channel' : 'Collapse channel'}
+                              disabled={isLastExpanded}
+                              title={
+                                isCollapsed
+                                  ? 'Expand channel'
+                                  : isLastExpanded
+                                  ? 'Cannot collapse last channel'
+                                  : 'Collapse channel'
+                              }
                             >
                               {isCollapsed ? <VscChevronDown size={16} /> : <VscChevronUp size={16} />}
                             </button>

benchmesh-serial-service/frontend/src/ui/workbench/EditorArea/editor.css

@@ -241,11 +241,16 @@
   transition: all 0.2s ease;
 }
 
-.channel-collapse-toggle:hover {
+.channel-collapse-toggle:hover:not(:disabled) {
   opacity: 1;
   background-color: var(--activity-bar-hover);
 }
 
+.channel-collapse-toggle:disabled {
+  opacity: 0.3;
+  cursor: not-allowed;
+}
+
 /* Collapsed channel state */
 .channel-column--collapsed {
   flex: 0 0 40px !important;