Environment

Vuetify Version: 3.11.2
Vue Version: 3.5.18
OS: macOS 10.15.7 (current)

Steps to reproduce

1. Open the Vuetify Playground: https://play.vuetifyjs.com/#...

2. Observe the initial state of the data table:

   • Group A contains 3 items: Item 1 (selectable), Item 2 (NOT selectable), Item 3 (selectable)
   • Group B contains 3 items: Item 4 (selectable), Item 5 (NOT selectable), Item 6 (NOT selectable)

3. Test the main "Select All" checkbox behavior (works correctly):

   • Click the "Select All" checkbox in the table header
   • Note that only 3 items are selected: Item 1, Item 3, and Item 4
   • Items 2, 5, and 6 remain unselected because they have selectable set to false
   • This is the correct behavior
   • Unselect all items before proceeding

4. Test the group header checkbox behavior (does not work correctly):

   • Click the checkbox in the "Group A" header
   • Note that ALL 3 items in Group A are selected, including Item 2
   • Item 2 should not be selected because item-selectable returns false for it
   • Expected behavior: Only Item 1 and Item 3 should be selected

5. Verify the same issue occurs with Group B:

   • Click the checkbox in the "Group B" header
   • Note that ALL 3 items in Group B are selected, including Items 5 and 6
   • Items 5 and 6 should not be selected
   • Expected behavior: Only Item 4 should be selected

6. Conclusion:

   • The group header checkboxes ignore the item-selectable function
   • The main "Select All" checkbox correctly respects the item-selectable function
   • This creates an inconsistent user experience

Expected Behavior

When clicking the checkbox in a group header, only items where item-selectable returns true should be selected, matching the behavior of the main "Select All" checkbox in the table header.

In the reproduction example:

• Clicking "Group A" checkbox should select only Item 1 and Item 3 (2 items)
• Item 2 should remain unselected because item-selectable returns false for it

Actual Behavior

When clicking the checkbox in a group header, ALL items in that group are selected, regardless of the item-selectable function result.

In the reproduction example:

• Clicking "Group A" checkbox selects Item 1, Item 2, and Item 3 (all 3 items)
• Item 2 is incorrectly selected even though item-selectable returns false for it

This is inconsistent with the main "Select All" checkbox behavior, which correctly respects item-selectable.