UI: Fix Mask popover crumpled

Make popover wider and image ID widget full width, like in textures properties.
Not ideal but at least the image name can be read now, until the ID widget
gets a more compact redesign.

Fixes T67748

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -772,6 +772,7 @@ class VIEW3D_PT_stencil_projectpaint(View3DPanel, Panel):
     bl_context = ".imagepaint"  # dot on purpose (access from topbar)
     bl_label = "Mask"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_ui_units_x = 14
 
     @classmethod
     def poll(cls, context):
@@ -796,20 +797,20 @@ class VIEW3D_PT_stencil_projectpaint(View3DPanel, Panel):
         col = layout.column()
         col.active = ipaint.use_stencil_layer
 
+        col.label(text="Stencil Image")
+        col.template_ID(ipaint, "stencil_image", new="image.new", open="image.open")
+
         stencil_text = mesh.uv_layer_stencil.name if mesh.uv_layer_stencil else ""
-        split = col.split(factor=0.5)
+
+        col.separator()
+
+        split = col.split()
         colsub = split.column()
         colsub.alignment = 'RIGHT'
         colsub.label(text="UV Layer")
         split.column().menu("VIEW3D_MT_tools_projectpaint_stencil", text=stencil_text, translate=False)
 
-        # todo this should be combined into a single row
-        split = col.split(factor=0.5)
-        colsub = split.column()
-        colsub.alignment = 'RIGHT'
-        colsub.label(text="Stencil Image")
-        colsub = split.column()
-        colsub.template_ID(ipaint, "stencil_image", new="image.new", open="image.open")
+        col.separator()
 
         row = col.row(align=True)
         row.prop(ipaint, "stencil_color", text="Display Color")