Q:

CapForm bugged in V11?

Bug introduced in 10.4 and fixed in 11.1
This problem is specific to OS X

It appears on my system (OSX 10.10.5) that CapForm isn't behaving itself in V11. Initially I thought it was a problem with the combination of CapForm and VertexColors but it seems even weirder than that!
For demonstration consider the four combinations of either CapForm[Automatic] or CapForm["Round"] with either no vertex colors or VertexColors->{Red,Green}:
t = Thickness[0.2];    
pr = PlotRange -> {{-0.2, 1.2}, {-0.2, 0.2}};

GraphicsGrid[{{
   Graphics[{CapForm[Automatic], t, Line[{{0, 0}, {1, 0}}]}, pr],
   Graphics[{CapForm[Automatic], t, Line[{{0, 0}, {1, 0}}, VertexColors -> {Red, Green}]}, pr]
 }, {
   Graphics[{CapForm["Round"], t, Line[{{0, 0}, {1, 0}}]}, pr],
   Graphics[{CapForm["Round"], t, Line[{{0, 0}, {1, 0}}, VertexColors -> {Red, Green}]}, pr]
 }}
]

V10.3

V11

As you can see, the rounded caps are not produced in V11. 
However, perhaps even more bizarrely, if I run the third entry (rounded ends, no vertex colors) on it's own outside of GraphicsGrid it works fine:
Graphics[{t, CapForm["Round"], Line[{{0, 0}, {1, 0}}]}, pr]

V11

Bugged?
Addendum
Perhaps a hint at the origin of this oddity is that when exported to a vector graphics format the rounded ends reappear! Although the vertex coloring is mutilated as in past versions:
line = Graphics[{CapForm["Round"], t, Line[{{0, 0}, {1, 0}}, VertexColors -> {Red, Green}]}, pr];
Export["~/Desktop/img31.pdf", %]

A:

On 11.1 on mac osx.
t = Thickness[0.2];
pr = PlotRange -> {{-0.2, 1.2}, {-0.2, 0.2}};

GraphicsGrid[{
 {Graphics[{CapForm[Automatic], t, Line[{{0, 0}, {1, 0}}]}, pr], 
  Graphics[{CapForm[Automatic], t, 
   Line[{{0, 0}, {1, 0}}, VertexColors -> {Red, Green}]}, pr]}, 
 {Graphics[{CapForm["Round"], t, Line[{{0, 0}, {1, 0}}]}, pr], 
 Graphics[{CapForm["Round"], t, 
  Line[{{0, 0}, {1, 0}}, VertexColors -> {Red, Green}]}, pr]}}]

