Bug 1939948 - When extending a gradient beyond the first/last specified color stops, do not add interpolated stops between the specified stops and the newly-added ones at 0% and/or 100%. r=longsonr

Per spec[1],

"Before the first color stop, the gradient line is the color of the first color stop,
and after the last color stop, the gradient line is the color of the last color stop."

Thus, these areas (if they exist, i.e. if the specified color stops do not reach to the
ends of the gradient line being rendered) are simply solid areas of the first or last
color respectively.

In most cases, adding interpolated stops within these areas would be harmless, as they
would all be the same color. But in the case of longer-hue interpolation, this is not
the case, and if we allow interpolation of extra stops, they will create an additional
"cycle" of the gradient, outside the range for which it was actually defined.

To avoid this, we ensure that when extending the color line in this way, only the new
end stop is added, without any intermediate interpolated stops.

[1] https://drafts.csswg.org/css-images-4/#coloring-gradient-line

Differential Revision: https://phabricator.services.mozilla.com/D233216

Author: jfkthame

layout/painting/nsCSSRenderingGradients.h

@@ -67,18 +67,27 @@ class MOZ_STACK_CLASS ColorStopInterpolator {
       // This is never the case on SVG gradients as they only use shorter hue.
       //
       // See https://bugzilla.mozilla.org/show_bug.cgi?id=1885716 for more info.
+      uint32_t extraStops = 0;
       if (mExtend) {
+        // If we're extending, we just need a single new stop, which will
+        // duplicate the end being extended; do not create interpolated stops
+        // within in the extension area!
         if (i == 0) {
           startPosition = std::min(startPosition, 0.0f);
+          extraStops = 1;
         }
         if (i == iterStops - 1) {
           endPosition = std::max(endPosition, 1.0f);
+          extraStops = 1;
         }
       }
-      uint32_t extraStops =
-          (uint32_t)(floor(endPosition * kFullRangeExtraStops) -
-                     floor(startPosition * kFullRangeExtraStops));
-      extraStops = std::clamp(extraStops, 1U, kFullRangeExtraStops);
+      if (!extraStops) {
+        // Within the actual gradient range, figure out how many extra stops
+        // to use for this section of the gradient.
+        extraStops = (uint32_t)(floor(endPosition * kFullRangeExtraStops) -
+                                floor(startPosition * kFullRangeExtraStops));
+        extraStops = std::clamp(extraStops, 1U, kFullRangeExtraStops);
+      }
       float step = 1.0f / (float)extraStops;
       for (uint32_t extraStop = 0; extraStop <= extraStops; extraStop++) {
         auto progress = (float)extraStop * step;