fix: blob corner fill snap + visual artifacts

plugin/src/Caelestia/Blobs/blobshape.cpp

@@ -32,6 +32,18 @@ static float cpuSmoothstep(float edge0, float edge1, float x) {
     return t * t * (3.0f - 2.0f * t);
 }
 
+static float cornerFillFactor(float sd, float smoothFactor) {
+    // Continuous two-sided window. The corner is squared (factor -> 0) only within
+    // ±smoothFactor of the neighbour's edge (the visible junction); it keeps its full
+    // radius both far outside the neighbour and deep inside it (where it is buried and
+    // squaring would only crease the interior). C0-continuous across sd = 0 — unlike the
+    // old `if (sd >= 0)` branch, which snapped the radius full<->square (factor 1<->0) on
+    // sub-pixel motion as a corner crossed the edge, flickering the fill bridge in/out.
+    const float outside = cpuSmoothstep(0.0f, smoothFactor, sd);  // 0 at edge, ->1 far outside
+    const float inside = cpuSmoothstep(0.0f, -smoothFactor, sd);  // 0 at edge, ->1 deep inside
+    return std::max(outside, inside);
+}
+
 BlobShape::BlobShape(QQuickItem* parent)
     : QQuickItem(parent) {
     setFlag(ItemHasContents);
@@ -310,20 +322,16 @@ void BlobShape::updatePolish() {
             if (si->isCornerExcluded(sj) || sj->isCornerExcluded(si))
                 continue;
             const auto& rj = m_cachedRects[j];
-            // Skip when the corner is inside rj: it's buried, not on a visible junction,
+            // Square each corner only near rj's edge; keep full radius far outside AND
-            // so squaring it would only perturb interior SDF gradients.
+            // deep inside rj (buried, so it can't crease the visible junction).
             const float sdTr = cpuSdBox(cTrX, cTrY, rj.cx, rj.cy, rj.hw, rj.hh);
             const float sdBr = cpuSdBox(cBrX, cBrY, rj.cx, rj.cy, rj.hw, rj.hh);
             const float sdBl = cpuSdBox(cBlX, cBlY, rj.cx, rj.cy, rj.hw, rj.hh);
             const float sdTl = cpuSdBox(cTlX, cTlY, rj.cx, rj.cy, rj.hw, rj.hh);
-            if (sdTr >= 0.0f)
+            fTr = std::min(fTr, cornerFillFactor(sdTr, smoothFactor));
-                fTr = std::min(fTr, cpuSmoothstep(0.0f, smoothFactor, sdTr));
+            fBr = std::min(fBr, cornerFillFactor(sdBr, smoothFactor));
-            if (sdBr >= 0.0f)
+            fBl = std::min(fBl, cornerFillFactor(sdBl, smoothFactor));
-                fBr = std::min(fBr, cpuSmoothstep(0.0f, smoothFactor, sdBr));
+            fTl = std::min(fTl, cornerFillFactor(sdTl, smoothFactor));
-            if (sdBl >= 0.0f)
-                fBl = std::min(fBl, cpuSmoothstep(0.0f, smoothFactor, sdBl));
-            if (sdTl >= 0.0f)
-                fTl = std::min(fTl, cpuSmoothstep(0.0f, smoothFactor, sdTl));
         }
 
         if (cornerFill && m_cachedHasInverted) {

plugin/src/Caelestia/Blobs/shaders/blob.frag

@@ -42,9 +42,12 @@ float sdBox(vec2 p, vec2 center, vec2 halfSize) {
 
 float smin(float a, float b, float k) {
     // Circular smooth min — the blend fillet is a true circular arc of radius k,
-    // tangent to both surfaces (not a polynomial/squircle curve). Deviation region
+    // tangent to both surfaces (not a polynomial/squircle curve). Deviates from
-    // width = k (deviates only where both a < k and b < k), matching the cubic it
+    // min(a, b) only in the corner region where BOTH a < k and b < k (unlike the
-    // replaces. Always <= min(a, b); max blend depth at a == b is (sqrt(2) - 1) * k.
+    // cubic it replaced, which deviated over the whole band |a - b| < k). Always
+    // <= min(a, b); max blend depth at a == b is (sqrt(2) - 1) * k. It is C1 but
+    // not C2 at the support boundary, so a circular-arc fillet shows the usual
+    // line-meets-arc curvature step — by design, that is the "circular" look.
     return max(k, min(a, b)) - length(max(vec2(k) - vec2(a, b), vec2(0.0)));
 }
 
@@ -164,8 +167,8 @@ void main() {
                 continue;
             if ((excludeMask & (1 << j)) != 0)
                 continue;
-            // smin only deviates from min within smoothFactor
+            // Circular smin deviates from min only where BOTH dArr are < smoothFactor.
-            if (abs(dArr[i] - dArr[j]) >= smoothFactor)
+            if (max(dArr[i], dArr[j]) >= smoothFactor)
                 continue;
             mergedSdf = min(mergedSdf, smin(dArr[i], dArr[j], smoothFactor));
         }
@@ -231,7 +234,16 @@ void main() {
 
         dInner -= sinkValue;
 
-        float dFrame = smaxSharpA(dOuter, -dInner, smoothFactor);
+        // The circular smax fillet has radius kFrame; when it exceeds the border thickness
+        // it can't complete inside the border, so the sharp outer-box term bleeds onto the
+        // inner edge and bulges the inner corners (worst when thickness < smoothFactor — the
+        // default border is thinner than the blend radius). Clamp kFrame to the thinnest side
+        // so the inner edge stays a clean constant-radius arc. Each inner corner is bounded by
+        // its thinner adjacent side, so the global min is correct for every corner.
+        float minThick = min(min(innerTop - outerTop, outerBot - innerBot),
+                             min(innerLeft - outerLeft, outerRight - innerRight));
+        float kFrame = clamp(min(smoothFactor, minThick - 1.0), 1.0, smoothFactor);
+        float dFrame = smaxSharpA(dOuter, -dInner, kFrame);
 
         mergedSdf = smin(mergedSdf, dFrame, smoothFactor);
         if (dFrame < minDist) {