Stop palette switches flashing a magnified texture

Switching palettes dropped the last frame's timing, so the first frame
fell back to one sample per ~29 pixels and drew a texture pixel in each
square. The first frame is now sized from the previous source's speed,
and coarse frames use block colours instead of texture pixels.

Model used: Claude Opus 5.5

core/src/main/java/art/arcane/iris/studio/map/NoiseExplorerGUI.java

@@ -99,6 +99,7 @@ public final class NoiseExplorerGUI extends JPanel {
     private static final int REFINEMENT_DELAY_MILLIS = 180;
     private static final long DEFAULT_SEED = 12345L;
     private static final double TEXTURE_PIXELS_MAX_BLOCKS_PER_PIXEL = 0.25D;
+    private static final int FULL_DETAIL_SAMPLE_STEP = 1;
     private static final double PALETTE_START_BLOCKS_PER_PIXEL = 1D / 16D;
     private static final long COARSE_SAMPLE_BUDGET = 4_096L;
     private static final double REFINEMENT_TARGET_MILLIS = 250D;
@@ -176,6 +177,7 @@ public final class NoiseExplorerGUI extends JPanel {
     private PaletteLayerPreview palettePreview;
     private int paletteDepth;
     private NoiseRenderCoordinator.Result renderResult;
+    private NoiseRenderCoordinator.Result lastTimedFrame;
     private NoiseRenderCoordinator.Request interactiveFrame;
     private boolean interactiveFramePending;
     private Point pointer;
@@ -991,12 +993,17 @@ public final class NoiseExplorerGUI extends JPanel {
     }
 
     private NoiseColorScale activeColorScale() {
+        return activeColorScale(FULL_DETAIL_SAMPLE_STEP);
+    }
+
+    // A coarse frame's sample stands for a whole square of pixels, so texture pixels would show magnified there.
+    private NoiseColorScale activeColorScale(int sampleStep) {
         PaletteLayerPreview preview = palettePreview;
         if (preview == null) {
             BlockRange blockRange = samplerBlockRange;
             return blockRange == null ? palette : new BlockHeightColorScale(palette, blockRange.lowest(), blockRange.highest());
         }
-        boolean showTexturePixels = viewport.blocksPerPixel() <= TEXTURE_PIXELS_MAX_BLOCKS_PER_PIXEL;
+        boolean showTexturePixels = viewport.blocksPerPixel() * sampleStep <= TEXTURE_PIXELS_MAX_BLOCKS_PER_PIXEL;
         return new PaletteLayerScale(paletteInspector.selectedView(), preview, palette, showTexturePixels);
     }
 
@@ -1106,9 +1113,10 @@ public final class NoiseExplorerGUI extends JPanel {
         return interactiveFrame != null && interactiveFrame.revision() == viewRevision;
     }
 
-    // The last frame's speed predicts this one, so cheap views draw sharp straight away instead of starting blocky.
+    // The last frame's speed predicts this one, so cheap views draw sharp straight away instead of starting blocky. It
+    // outlives a switch to another source, which samples about as fast.
     private long interactiveSampleBudget() {
-        NoiseRenderCoordinator.Result lastResult = renderResult;
+        NoiseRenderCoordinator.Result lastResult = lastTimedFrame;
         if (lastResult == null) {
             return COARSE_SAMPLE_BUDGET;
         }
@@ -1166,7 +1174,7 @@ public final class NoiseExplorerGUI extends JPanel {
                 viewRevision,
                 sampler,
                 viewport,
-                activeColorScale(),
+                activeColorScale(sampleStep),
                 getWidth(),
                 renderHeight(),
                 sampleStep
@@ -1680,6 +1688,7 @@ public final class NoiseExplorerGUI extends JPanel {
                 rendering = false;
                 errorMessage = null;
                 renderResult = result;
+                lastTimedFrame = result;
                 repaint();
                 refreshStatus();
                 if (finishInteractiveFrame(result.request())) {