The problem to fix is that the arrow drawn by draw_arrow1() in src/fl_symbols.cxx
displays a faint clear line between the stem and head of the arrow with the Cairo
graphics driver.
This occurs because draw_arrow1() draws the arrow in 2 steps (a rectangle +
a triangle) and the Cairo driver is configured to use antialiasing when filling
polygons. The antialiasing produces the faint line between stem and head.
Why does draw_arrow1() draw a rectangle + a triangle rather than a
7-vertex polygon? That's because the X11 graphics driver fails with its polygon-
drawing function when the polygon is also rotated: the polygon is drawn
empty.
We want to keep using antialiasing under Cairo for polygons because
the result is better with non horizontal/vertical polygon edges.
This implementation changes function draw_arrow1() which draws
the arrow as a 7-vertex filled polygon except when the graphics driver
returns false for its virtual member function can_fill_non_convex_polygon().
In that situation, draw_arrow1() draws, as before, a rectangle + a triangle.
The new, virtual member function can_fill_non_convex_polygon() returns
true except for the X11 graphics driver. Therefore, draw_arrow1() is effectively
unchanged under the X11 driver.
For a very simple test program see STR 3222, File 2:
https://www.fltk.org/strfiles/3222/value_slider.cxx
Known issues:
- range checking of the value box size is very limited
- resizing the widget does not change the value box size
- drawing issues are possible if the value box size and the widget
size are not compatible (e.g. value box too large)
This was necessary because the latest GitHub "runner" image (20221120.1)
includes incompatible image libs (libjpeg, libpng, and zlib). This
caused `fluid-cmd.exe` to fail with error code -1073741511. For further
information please refer to this GitHub Issue:
https://github.com/actions/runner-images/issues/6627
The previous runner version (20221027.1) did not include these libs
and our build used the internal libs and worked.
The fix is to disable the search for system libs in CMake.
Note: this does not explain *why* these libs are incompatible.
Fl_Flex would not change type when it had no children.
Function evaluation could read past string end.
Fl_Window_Type would not initialise all member variables.
