Great work, there are some good changes to this UI! I do have some suggestions:

1. Is it possible to add the animation decoration to the property values? I mean the icons on the right here:
   
2. I know Blender doesn't really have any designated icons for editing something, but I feel the edit property button should use a different icon, since the current one is already associated with modifiers in Blender. I tried some icons, and in my opinion TOOL_SETTINGS and GREASEPENCIL would work a lot better.

Nice tweaks! Not totally sold on removing the Library Override indicator, better check with @Bastien Montagne (mont29) as it must have been added for a reason? We can use the Library Override icon instead (DECORATE_LIBRARY_OVERRIDE).

Some suggestions:

• Rename Add to New with a plus icon. To match other places where we have a big New operator in panels.
• Separator between the New button and list.
• Use PREFERENCES icon instead of MODIFIER_ON.
• Do not align the values with the operator buttons. (since it makes sliders right-side not rounded)

Suggestions applied:

diff --git a/release/scripts/modules/rna_prop_ui.py b/release/scripts/modules/rna_prop_ui.py
index dcd883be274..b0acb661741 100644
--- a/release/scripts/modules/rna_prop_ui.py
+++ b/release/scripts/modules/rna_prop_ui.py
@@ -176,7 +176,7 @@ class PropertyPanel:
         else:
             value_column.prop(rna_item, '["%s"]' % escape_identifier(key), text="")
 
-        operator_row = value_row.row(align=True)
+        operator_row = value_row.row()
 
         # Do not allow editing of overridden properties (we cannot use a poll function 
         # of the operators here since they's have no access to the specific property).
@@ -185,7 +185,8 @@ class PropertyPanel:
         if use_edit and is_rna:
             operator_row.label(text="API Defined")
         elif use_edit and not is_lib_override:
-                props = operator_row.operator("wm.properties_edit", text="", icon='MODIFIER_ON', emboss=False)
+                sub_row = operator_row.row(align=True)
+                props = operator_row.operator("wm.properties_edit", text="", icon='PREFERENCES', emboss=False)
                 props.data_path = context_member
                 props.property_name = key
                 props = operator_row.operator("wm.properties_remove", text="", icon='X', emboss=False)
@@ -214,9 +215,11 @@ class PropertyPanel:
         # TODO: Allow/support adding new custom props to overrides.
         if use_edit and not is_lib_override:
             row = layout.row()
-            props = row.operator("wm.properties_add", text="Add")
+            props = row.operator("wm.properties_add", text="New", icon="ADD")
             props.data_path = context_member
             del row
+        
+        layout.separator()
 
         show_developer_ui = context.preferences.view.show_developer_ui
         rna_properties = {prop.identifier for prop in rna_item.bl_rna.properties if prop.is_runtime} if items else None

Not necessarily layout, but a couple suggestions (perhaps a bit outside the scope of this patch).

1. Clicking on New should open the edit dialog, preferably with the cursor in the Name field to start typing right away (similar to the Rename operator or Move to Collection)
2. A button next to Edit and X, to reset the property to its default value.
   
3. Just a thought: for the property name, have you tried to have a text field instead of a label? It'd make editing faster but more prone to renaming props by mistake.

Tested, certainly a huge improvement!

A downside of one of the changes is that the value of unsupported types isn't directly visible anymore.

I use unsupported types a lot, and I think this is not a big issue.

Keep in mind that custom properties are also drawn in the N panel, which is currently broken. (but it seems this is the same issue that Aaron pointed out)

Other than that, LGTM!

I did run into a couple little artifacts but I think those have nothing to do with this patch, but here they are anyways:

• The last letter of "cloudrig_parameters" is a bit shy. But hey, who doesn't want to hide from the world every now and then: F11001227
• When editing a massive piece of data(21200 characters), the text box gets very confused and doesn't scroll to where the cursor is. The cursor is allowed to step a few steps outside the box before disappearing: F11001717
  • Note that the old Edit popup's text field would be capped at 1024 characters and could result in loss of data if not careful, so the way in which it's broken with the patch is actually an improvement over the way in which it's broken in master.

RE override text, this was added to 'keep same space' as non-overridden cases, and also to explicitly differentiate (in the future) between locale customprops added to the override, from those coming from its linked reference. Am totally fine with using e.g. the grayed out override icon instead of the editing one, and also probably the grayed out cross?

Thanks for all of the testing and feedback!

Is it possible to add the animation decoration to the property values? I mean the icons on the right here:

Yeah, it's possible, but to me the three icon buttons on the right is just a bit too much.

Rename Add to New with a plus icon. To match other places where we have a big New operator in panels.

I'm not sure this is more common honestly, I feel like I've mostly seen "Add" without the icon in the past. I don't have a strong opinion though, so I added it.

Separator between the New button and list.

Sure, that looks good

Do not align the values with the operator buttons. (since it makes sliders right-side not rounded)

Good point!

The last letter of "cloudrig_parameters" is a bit shy. But hey, who doesn't want to hide from the world every now and then

I think that's a more general bug with the UI code from recent master-- I noticed the same thing in nodes.

Icons

Use PREFERENCES icon instead of MODIFIER_ON.

Hmm, I'm not sure about this one. We had discussed using the preferences icon for whenever something from Blender's preferences is exposed in the UI elsewhere, maybe it's best to save it for that?
The preferences icon is very white, and takes a lot of visual weight too.

I tried some icons, and in my opinion TOOL_SETTINGS and GREASEPENCIL would work a lot better.

Reusing icons like this is pretty awkward. I don't think there's any very convincing solution. I like the tool settings icon a bit better, but it's also a bit busy.
I think it's a little worse to borrow the icon from a data type like grease pencil, I'm not sure though.

I'm not thrilled with any of the options here really. Personally I prefer MODIFIER_ON because of its simplicity. but my opinion isn't that strong.

In D12805#335288, @RedMser (RedMser) wrote:

Great work, there are some good changes to this UI! I do have some suggestions:

1. Is it possible to add the animation decoration to the property values? I mean the icons on the right here:
   

@Hans Goudey (HooglyBoogly) is there a technical reason why decorators cannot be used here?

2. I know Blender doesn't really have any designated icons for editing something, but I feel the edit property button should use a different icon, since the current one is already associated with modifiers in Blender. I tried some icons, and in my opinion TOOL_SETTINGS and GREASEPENCIL would work a lot better.

I agree a dedicated EDIT icon should be added however, I do not think that is a show stopper here. I we have a task somewhere to request new icons. A User Interface module member will know more here.

I am still getting this import error:

  File "/home/aaron/Documents/projects/blender_foundation/blender/build_linux/bin/3.0/scripts/addons/rigify/utils/bones.py", line 24, in <module>
    from rna_prop_ui import rna_idprop_ui_prop_get
ImportError: cannot import name 'rna_idprop_ui_prop_get' from 'rna_prop_ui' (/home/aaron/Documents/projects/blender_foundation/blender/build_linux/bin/3.0/scripts/modules/rna_prop_ui.py)

@Hans Goudey (HooglyBoogly) is there a technical reason why decorators cannot be used here?

I responded to that in my last comment:

Yeah, it's possible, but to me the three icon buttons on the right is just a bit too much.

In other words, I personally don't really want to add them in this patch since I'm not sure about it myself. Could be added later though if people wanted. Or if I was convinced otherwise now.

Thanks for the report about the import error.

In D12805#338212, @Aaron Carlisle (Blendify) wrote:

I am still getting this import error:

  File "/home/aaron/Documents/projects/blender_foundation/blender/build_linux/bin/3.0/scripts/addons/rigify/utils/bones.py", line 24, in <module>
    from rna_prop_ui import rna_idprop_ui_prop_get
ImportError: cannot import name 'rna_idprop_ui_prop_get' from 'rna_prop_ui' (/home/aaron/Documents/projects/blender_foundation/blender/build_linux/bin/3.0/scripts/modules/rna_prop_ui.py)

It looks like arc is resetting submodules to commits from few months ago when you use arc diff, that import is not used in master.