There are many instances where customizing the header row is desired so
return both the header row layout and the main body layout.
For consistency and correctness, the arrow symbol and the is_open check
are always performed so callers only need to perform layout tasks.
The C++ side now receives a `PanelLayout` structure containing both:
```cpp
PanelLayout panel_layout = uiLayoutPanelWithHeader( ... );
uiItemL(panel_layout.header, ... );
if (panel_layout.body) {
...
}
```
And the Python side receives a tuple:
```python
header, body = layout.panel( ... )
header.label(...)
if body:
...
```
```python
import bpy
from bpy.props import BoolProperty
class LayoutDemoPanel(bpy.types.Panel):
bl_label = "Layout Panel Demo"
bl_idname = "SCENE_PT_layout_panel"
bl_space_type = 'PROPERTIES'
bl_region_type = 'WINDOW'
bl_context = "scene"
def draw(self, context):
layout = self.layout
scene = context.scene
layout.label(text="Basic Panels")
header, panel = layout.panel("my_panel_id", default_closed=True)
header.label(text="Hello World")
if panel:
panel.label(text="Success")
header, panel = layout.panel_prop(scene, "show_demo_panel")
header.label(text="My Panel")
if panel:
panel.prop(scene, "frame_start")
panel.prop(scene, "frame_end")
layout.label(text="Customized headers")
# Add a checkbox to the Panel header. text must be None for this panel
header, panel = layout.panel("my_panel_id-2", default_closed=True)
header.prop(scene, "use_checkmark", text="") # text must be empty for the checkbox
header.label(text="Checkmark at beginning")
if panel:
panel.label(text="Success")
header, panel = layout.panel("my_panel_id-3", default_closed=True)
header.label(text="Buttons at the end")
header.operator("mesh.primitive_cube_add", text="", icon='EXPORT')
header.operator("mesh.primitive_cube_add", text="", icon='X')
if panel:
panel.label(text="Success")
header, panel = layout.panel("my_panel_id-4", default_closed=True)
header.prop(scene, "use_checkmark2", text="")
header.label(text="Both")
header.operator("mesh.primitive_cube_add", text="", icon='EXPORT')
header.operator("mesh.primitive_cube_add", text="", icon='X')
if panel:
panel.label(text="Success")
bpy.utils.register_class(LayoutDemoPanel)
bpy.types.Scene.show_demo_panel = BoolProperty(default=False)
bpy.types.Scene.use_checkmark = BoolProperty(default=False)
bpy.types.Scene.use_checkmark2 = BoolProperty(default=False)
```
Pull Request: https://projects.blender.org/blender/blender/pulls/117248
ddd06eeb8f