bugfix [6051a9fc]: MS-Win canvas arcs with to small extend are drawn as 360 degrees

Author: oehhar

changes.md

@@ -26,6 +26,7 @@ to the userbase.
  - [Aqua: compiler warning in tkMacOSXImage.c](https://core.tcl-lang.org/tk/tktview/7f3751)
  - [tkpWinRopModes[GXnoop] is R2_NOT, should be R2_NOP](https://core.tcl-lang.org/tk/tktview/95da0f)
  - [image svg: support paint order and fix possible nan race condition](https://core.tcl-lang.org/tk/tktview/b43dbc)
+ - [MS-Win canvas arcs with small extent are drawn 360 dregrees](https://core.tcl-lang.org/tk/info/6051a9fc)
 
 Release Tk 9.0.2 arises from the check-in with tag `core-9-0-2`.
 

win/tkWinDraw.c

@@ -1213,6 +1213,13 @@ DrawOrFillArc(
     int clockwise = (extent < 0); /* non-zero if clockwise */
     int xstart, ystart, xend, yend;
     double radian_start, radian_end, xr, yr;
+    int extent_less_than_180_deg;
+
+    extent = extent % (64*360);
+    if (extent < 0) {
+	extent += (64*360);
+    }
+    extent_less_than_180_deg = (extent < (64*180));
 
     if (d == None) {
 	return BadDrawable;
@@ -1256,6 +1263,32 @@ DrawOrFillArc(
     xend = (int)((xr + cos(radian_end)*width/2.0) + 0.5);
     yend = (int)((yr + sin(-radian_end)*height/2.0) + 0.5);
 
+    if ((xstart == xend) && (ystart == yend) && extent_less_than_180_deg) {
+        /*
+         * The extent is so small that the arc size is less than one pixel.
+         * If the Arc, Chord, or Pie GDI function later received this, then
+         * a complete ellipse would be drawn instead of the desired 1-pixel
+         * size arc. The end point must be made different from the start
+         * point. Since (at this level in the code) arcs are always drawn
+         * counterclockwise, either xend or yend needs adjustment, depending
+         * on the sub-range where radian_start lies (it was constrained to
+         * the [0 ; 2*PI[ range earlier). See bug [6051a9fc]
+         */
+        if (radian_start > PI/4) {
+            if (radian_start < 3*PI/4) {
+                xend--;
+            } else if (radian_start < 5*PI/4) {
+                yend++;
+            } else if (radian_start < 7*PI/4) {
+                xend++;
+            } else {
+                yend--;
+            }
+        } else {
+            yend--;
+        }
+    }
+
     /*
      * Now draw a filled or open figure. Note that we have to increase the
      * size of the bounding box by one to account for the difference in pixel