I love Synfig, i find it amazing and very useful, with many abilities and potentials .
But i think Synfig needs a new and modern UI.
Thanks for your efforts.

— Maisam Hosaini (@maisam_hosaini) December 28, 2019

Below attached zip is my rough basic idea for new synfig user interface design:

Concept from Builder.zip

Header bar implementation is the main focus since it makes the GTK app look modern and saves more space for content. Moreover, Cinnamon, KDE, and XFCE are already on board with GTK3 so header bar will appear consistent on all major desktop environments.
Below is a screenshot of gedit (with header bar) on KDE Neon:

The only drawback would be deprecating traditional menu bar, reorganizing preferences, headache from implementers ;P and other documentation related stuff (because new UI equals new guides, tutorials, instructions,...)

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