Compact Menu applet icon size is hardcoded and prevents full-size panel button theming

[own3mall]

Steps to Reproduce

Environment:

• Ubuntu 26.04
• MATE 1.28.2
• GTK 3.20 theme

Component:
mate-panel Compact Menu applet

Description:
The Compact Menu launcher button appears to force the menu icon
(start-here.png/start-here-symbolic.png) to a 24x24 allocation.

A custom GTK theme can style the Brisk Menu launcher because it exposes
button.brisk-button and allows a background image on the button's child box.

The Compact Menu applet exposes only:

PanelWidget
└── button:dir-ltr

and appears to allocate the icon internally as a menu-sized icon.

Expected behavior:
The launcher button should respect the image/icon allocation or provide a
CSS style class/node that allows themes to replace the icon with a custom
sized launcher background.

Example:
A 101x30 PNG used as the launcher asset should be able to fill a 101x30
panel button without being scaled down to 24x24.

Steps to reproduce:

1. Add Compact Menu applet to panel.
2. Replace start-here.png with a 101x30 image.
3. Observe the image is scaled to approximately 24x24.
4. Attempt to override with GTK CSS:
   -gtk-icon-size
   min-width/min-height
   button background-image
   PanelWidget button selectors
5. No effect on Compact Menu icon allocation.

Expected Behavior

Allow compact menu to be styled so a different image can be used that is not constrained to 24x24 pixels. I want to use the old XP start button in my theme for the compact menu which isn't currently possible, but you can style the brisk button.

Actual Behavior

Can't style compact menu at all or use an image larger than 24x24 pixels.

Error Output

Version

MATE 1.28.2 - GTK 3.20 theme

Distribution

Ubuntu 26.04

Additional Context

No response

Reproducibility

Every time

Checklist

• I have searched existing issues to avoid duplicates
• I have provided all the requested information above