[Impact]
Humanity icons are sometimes displayed too big on hidpi screens in 16.04. On both 16.04 and 16.10, different icons are used on hidpi screens than non-hidpi screens. Sometimes the icons look similar but with much thinner strokes, while other times an icon with a different metaphor is used. Users on hidpi screens will notice that icons in applications like Nautilus, Transmission, etc. look different (not just clearer) on hidpi screens.
The fix here is to add support for @2 icons to the Humanity theme by adding symbolic links for @2 icons and updating the index.theme files accordingly.
We should backport the fix to 16.04 and 16.10, so Humanity icons look the same on hidpi screens as non-hidpi screens.
[Test Case]
Here are a few test cases:
1) Using a computer with hidpi screen and a computer with a lowdpi screen both running Ubuntu 16.04: Open Nautilus and see that the icons on the hidpi screen are much larger. (This applies to icons on the desktop as well.)
-The fix should result in icons being the same size on both hidpi and lowdpi.
2) Using a computer with a hidpi screen and one with a lowdpi screen running Ubuntu 16.04 or 16.10: Open Transmission and see that the "Open" button in the toolbar uses an icon with a different metaphor on hidpi screens (an up arrow emerging from a folder) than on lowdpi screens (a sheet of paper emerging from a folder).
- The fix should result in the "sheet of paper emerging from a folder" icon being used here on both hidpi and lowdpi screens.
3) Using a computer with hidpi screen and a computer with a lowdpi screen both running Ubuntu 16.10: Compare icons in Nautilus and see that the icons look sutbly different. The strokes in icons on the hidpi screen will be visibly thinner on the hidpi screen because icons for bigger scale are used (say 48 on hidpi instead of 24) rather than rendering the original size icon at a larger resolution.
- The fix should result in icons that have the same stroke width on hidpi and lowdpi screens. The icons will still be sharper on the hidpi screen, but the lines won't be thinner.
[Regression Potential]
The patch is designed for compatibility where @2 support is not available by listing the @2 directories under "ScaledDirectories=" rather than "Directories=". However it's possible there could be other kinds of issues not found in testing. If there are any omissions or errors in the index.theme, this could cause rendering issues for some categories of icons. Comparing icons on a broad range of applications and systems should catch this class of issues.
Hidpi users may find that their machine looks different after the update than it did before. This is not a regression, but a fix to a bug that's been present all along.
Left side: humanity without @2x support
Right side: humanity with @2x support (looks just like lowdpi but crisper)