Screens with higher pixel densities and resolutions are becoming more common, especially among artists. This is what Pencil2D currently looks like on a 4k display:

There are issues all over the place, from clipped text in the timeline to a minuscule toolbox to a rather small color palette (that is with the large option!) and color wheel. If I'm not mistaken there were multiple issues related to this in the forums, but to my surprise I could not find an issue on it on Github. So this issue will be for tracking all of the changes that need to be done to make Pencil2D work just as well on high resolution displays, if not better.

See here for some details on Qt and High DPI support: https://doc.qt.io/qt-5/highdpi.html