Fix CairoMakie line color bug #3704 (#3712)

* Fix #3704

* add reference test

* add changelog entry

CHANGELOG.md

@@ -1,8 +1,8 @@
 # Changelog
 
 ## [Unreleased]
-- Added supported markers hint to unsupported marker warn message.
-
+- Added supported markers hint to unsupported marker warn message [#3666](https://github.com/MakieOrg/Makie.jl/pull/3666).
+- Fixed bug in CairoMakie line drawing when multiple successive points had the same color [#3712](https://github.com/MakieOrg/Makie.jl/pull/3712).
 - Remove StableHashTraits in favor of calculating hashes directly with CRC32c [#3667](https://github.com/MakieOrg/Makie.jl/pull/3667).
 
 ## [0.20.8] - 2024-02-22

CairoMakie/src/primitives.jl

@@ -257,6 +257,13 @@ function draw_multi(primitive::Lines, ctx, positions, colors::AbstractArray, lin
                 end
             else
                 prev_continued = false
+
+                # finish previous line segment
+                Cairo.set_line_width(ctx, prev_linewidth)
+                !isnothing(dash) && Cairo.set_dash(ctx, dash .* prev_linewidth)
+                Cairo.set_source_rgba(ctx, red(prev_color), green(prev_color), blue(prev_color), alpha(prev_color))
+                Cairo.stroke(ctx)
+
                 if !this_nan
                     this_linewidth != prev_linewidth && error("Encountered two different linewidth values $prev_linewidth and $this_linewidth in `lines` at index $(i-1). Different linewidths in one line are only permitted in CairoMakie when separated by a NaN point.")
                     # this is not nan

ReferenceTests/src/tests/primitives.jl

@@ -36,6 +36,10 @@ end
     s
 end
 
+@reference_test "lines issue #3704" begin
+    lines(1:10, sin, color = [fill(0, 9); fill(1, 1)], linewidth = 3, colormap = [:red, :cyan])
+end
+
 @reference_test "scatters" begin
     s = Scene(size = (800, 800), camera = campixel!)