fix(border-color): keep ComboBox popup clicks inside LeftPanel (TASK 63)

TASKS/task-63-border-colour-app-dropdown.md

@@ -31,6 +31,17 @@ not *"is this control type themed?"*.
   matches `SelectedValue` ordinally (case-sensitively) and the palette mixes `#FFFFFF` and
   `#e94560`; the model matches by VALUE, so the hex the creator typed in any case still lands
   on the right row.
+- **Clicking a popup row stays inside `LeftPanel` (2026-10-22 fix):** a `ComboBox`'s open
+  `PART_Popup` lives in its own `PopupRoot` visual tree (`VisualTreeHelper.GetParent(popupRoot)`
+  is `null`), connected back to the `Popup` and `ComboBox` only via `PopupRoot`'s **logical**
+  parent (`LogicalTreeHelper.GetParent(popupRoot)`). `LeftPanel.IsDescendantOf` previously used
+  `child is Visual ? VisualTreeHelper.GetParent(child) : LogicalTreeHelper.GetParent(child)`,
+  which returned `null` at `PopupRoot` because `PopupRoot` is a `Visual`. That caused
+  `MainWindow.Window_PreviewMouseLeftButtonDown` to see `IsClickInside == false` on any click
+  inside the open dropdown and clear `SelectedElement = null` — exiting the properties panel
+  before the `ComboBoxItem` click could select the colour. `IsDescendantOf` now falls back to
+  `LogicalTreeHelper.GetParent` and `TemplatedParent` whenever `VisualTreeHelper.GetParent`
+  returns `null`.
 - **The closed box shows the swatch + the colour's NAME; the rows show swatch + hex + name**
   (the rows the creator approved in TASK 60). A ComboBox cannot do that with one
   `ItemTemplate` — WPF copies it into the read-only `SelectionBoxItemTemplate` — so