Degraded Mesh Gradient Editing Performance when the "Selectors and CSS" panel is shown

Summary:

After wondering why mesh gradient editing suddenly became unusably slow on my machine while still working fine on a less capable laptop, I was able to trace it down by successively replacing config files from one machine (working) to the other (slow). It turned out that it's the existence of the "Selectors and CSS" panel which is causing the degradation. It doesn't need to be the active/visible panel - it's sufficient when it's part of the current window layout.

The mentioned degradation manifests in a way that when dragging around any of the gradient mesh nodes, visual updates are occurring every 1-5 seconds only. Closing that panel returns behavior back to normal: dragging mesh nodes provides a fluent visual feedback where the view is re-drawn many times per second.

Steps to reproduce:

• Reset Inkscape settings
• Open Inkscape
• Draw a rectangle
• Apply a mesh gradient fill with a few rows and columns
• Choose the mesh gradient tool and drag around some nodes => works fluently
• Show the "Selectors and CSS" panel
• Try draging mesh gradients again => slow visual updating

Version info

Inkscape 1.3.2 (091e20e, 2023-11-25, custom)

    GLib version:     2.78.1
    GTK version:      3.24.39
    glibmm version:   2.66.6
    gtkmm version:    3.24.8
    libxml2 version:  2.11.5
    libxslt version:  1.1.38
    Cairo version:    1.18.0
    Pango version:    1.50.14
    HarfBuzz version: 8.2.2

    OS version:       Windows 11 23H2