Can you look into why this happens?

I will give it a try.

Since I'm not familiar with either TypeScript or the vscode structure, I don't have a fix, only some random findings.

Inspecting the 2 HTML elements (the view/title submenu which does have an icon and the multiDiffEditor/resource/title which does not):

• the working one has an a element with class="action-label codicon-squirrel codicon";
• the broken one has an a element with class="action-label submenu codicon" (note the submenu instead of codicon-...);

For both of these, we eventually construct a DropdownMenuActionViewItem, but the call trace for getting there is a bit different:

1. view/title rendering (working fine)

• WorkbenchToolBar in created here with an actionViewItemProvider that calls getActionViewItem;
• getActionViewItem calls createActionViewItem here
• createActionViewItem creates an instance of SubmenuEntryActionViewItem here
• classNames is set to the expected value based on the item.icon object before calling the super (DropdownMenuActionViewItem) constructor:

  vscode/src/vs/platform/actions/browser/menuEntryActionViewItem.ts

  Line 300 in 0d0ddc5

  classNames: options?.classNames ?? (ThemeIcon.isThemeIcon(action.item.icon) ? ThemeIcon.asClassName(action.item.icon) : undefined),

2. multiDiffEditor/resource/title (not working fine)

• MenuWorkbenchToolBar is created here
• MenuWorkbenchToolBar constructor calls the constructor of the super (WorkbenchToolBar) here
• WorkbenchToolBar constructor calls the constructor of the super (ToolBar) here
• the ToolBar constructor can set a classname for the "More" codicon here, but doesn't know what to do for our submenu:

  vscode/src/vs/base/browser/ui/toolbar/toolbar.ts

  Line 115 in 0d0ddc5

  classNames: action.class,

• so we end up with an untouched class name ("submenu") that was set at the beginning:

  vscode/src/vs/platform/actions/common/actions.ts

  Line 430 in 0d0ddc5

  super(`submenuitem.${item.submenu.id}`, typeof item.title === 'string' ? item.title : item.title.value, actions, 'submenu');

The difference between the two is that the first one creates an instance of SubmenuEntryActionViewItem (which extends DropdownMenuActionViewItem), while the second creates directly a DropdownMenuActionViewItem.

Now, given the separation between base and platform, I'm not sure whether we should just handle the classes for the submenu along the calls in platform or not. But once we get to the ToolBar class in base, we no longer hold a reference to the item that contains the item.icon we are interested in.

What does work and is probably not how we should handle this is setting the class name from the start in

-super(`submenuitem.${item.submenu.id}`, typeof item.title === 'string' ? item.title : item.title.value, actions, 'submenu');
+super(`submenuitem.${item.submenu.id}`, typeof item.title === 'string' ? item.title : item.title.value, actions, ThemeIcon.isThemeIcon(item?.icon) ? ThemeIcon.asClassName(item.icon) : 'submenu');

Like I said, I'm a bit out of my depth here, I'm not familiar with the design and how this should be handled, I'm just writing this down in case it helps.

Thank you, that is a very detailed analysis! Will look into it