feat: scrollbar thumb travel math + UI tweaks

- Scrollbars: proper track-travel geometry so a min-size thumb stays
  inside the track and dragging maps 1:1 to the cursor at any zoom;
  clicking the empty track jumps the view to re-center the thumb under
  the cursor. Rounded thumbs with hover/active highlight; taller track.
- Spectrum slice (PSD) always spans the full frequency range; ignoring
  the selection's freq bounds avoids cropping bins and skewing the
  auto-ranged dB axis and peak pick. Selection rect still drawn normally.
- Default annotation overlay opacity 0.06 -> 0.24 (quiet but legible).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/render.c

@@ -714,9 +714,13 @@ void DrawSpectrumPanel(Rectangle bounds)
                    app.sel.freqStart > 0.001f || app.sel.freqEnd < 0.999f);
     float t0 = hasSel ? app.sel.timeStart : app.view.start;
     float t1 = hasSel ? app.sel.timeEnd   : app.view.end;
-    float f0 = hasSel ? app.sel.freqStart : app.view.freqStart;
+    // The spectrum slice always spans the complete frequency range. Honoring a
-    float f1 = hasSel ? app.sel.freqEnd   : app.view.freqEnd;
+    // partial frequency selection only crops the displayed bins and skews the
-    if (f1 < f0) { float t = f0; f0 = f1; f1 = t; }
+    // auto-ranged dB axis + peak pick (inaccurate bins), so we ignore the
+    // selection's freq bounds here. The selection rectangle is still drawn
+    // normally by DrawSelection(); we just don't filter the slice by frequency.
+    float f0 = 0.0f;
+    float f1 = 1.0f;
 
     int maxBins = FFT_SIZE_MAX / 2 + 1;
     float* power = (float*)malloc(maxBins * sizeof(float));

src/spectrogram.c

@@ -142,7 +142,7 @@ static Layout ComputeLayout(void)
     L.scale = GetUIScale();
     L.sidebarWidth = 320 * L.scale;
     L.labelHeight = 15 * L.scale;
-    L.scrollbarHeight = 18 * L.scale;
+    L.scrollbarHeight = 22 * L.scale;
     L.freqLabelWidth = 65 * L.scale;
     L.vScrollbarWidth = 18 * L.scale;
     L.topMargin = 50 * L.scale;
@@ -827,7 +827,7 @@ int main(int argc, char* argv[])
     app.displayMaxFreqHz = 0.0f;  // 0 = no crop; user sets via sidebar slider
     app.showAnnotations = true;
     app.annotationsExpanded = false;
-    app.annotationOpacityBase  = 0.06f;  // whisper-faint by default — signal wins
+    app.annotationOpacityBase  = 0.24f;  // quiet but legible by default — signal still wins
     app.annotationOpacityHover = 0.65f;  // pop on hover / selection
     // Optional CLI override of the resting overlay alpha (e.g. for a brighter
     // GUI default). The headless render path sets its own default separately.
@@ -1584,57 +1584,80 @@ int main(int argc, char* argv[])
                     viewBounds, (Vector2){ 0, 0 }, 0.0f, WHITE);
             }
 
-            // Draw scrollbars
+            // Draw scrollbars. Thumb geometry uses proper "track travel" math
+            // (thumb travels over width-thumb, not the whole track) so a min-size
+            // thumb stays inside the track and dragging maps 1:1 to the cursor at
+            // any zoom. minThumb keeps the thumb grabbable when zoomed way in.
+            Vector2 mouse = GetMousePosition();
+            float minThumb = 28.0f * renderScale;
+            Color trackColor  = (Color){ 40, 40, 46, 255 };
+            Color trackBorder = (Color){ 72, 72, 82, 255 };
+            Color thumbColor  = (Color){ 110, 114, 126, 255 };
+            Color thumbHot    = (Color){ 158, 164, 180, 255 };
+
             // Horizontal scrollbar (time)
-            DrawRectangleRec(hScrollbar, DARKGRAY);
+            float hViewW = app.view.end - app.view.start;
-            float hThumbWidth = (app.view.end - app.view.start) * hScrollbar.width;
+            float hThumbW = fmaxf(hViewW * hScrollbar.width, minThumb);
-            float hThumbX = hScrollbar.x + app.view.start * hScrollbar.width;
+            float hTravel = hScrollbar.width - hThumbW;
-            if (hThumbWidth < 10) hThumbWidth = 10;
+            float hDenom  = 1.0f - hViewW;
-            DrawRectangle(hThumbX, hScrollbar.y, hThumbWidth, hScrollbar.height, GRAY);
+            float hThumbX = hScrollbar.x + (hDenom > 1e-6f ? (app.view.start / hDenom) * hTravel : 0.0f);
+            Rectangle hThumb = { hThumbX, hScrollbar.y, hThumbW, hScrollbar.height };
 
-            // Vertical scrollbar (frequency)
+            // Vertical scrollbar (frequency; axis flipped so freqEnd=1 is at top)
-            DrawRectangleRec(vScrollbar, DARKGRAY);
+            float vViewH = app.view.freqEnd - app.view.freqStart;
-            float vThumbHeight = (app.view.freqEnd - app.view.freqStart) * vScrollbar.height;
+            float vThumbH = fmaxf(vViewH * vScrollbar.height, minThumb);
-            float vThumbY = vScrollbar.y + (1.0f - app.view.freqEnd) * vScrollbar.height;
+            float vTravel = vScrollbar.height - vThumbH;
-            if (vThumbHeight < 10) vThumbHeight = 10;
+            float vDenom  = 1.0f - vViewH;
-            DrawRectangle(vScrollbar.x, vThumbY, vScrollbar.width, vThumbHeight, GRAY);
+            float vThumbY = vScrollbar.y + (vDenom > 1e-6f ? ((1.0f - app.view.freqEnd) / vDenom) * vTravel : 0.0f);
+            Rectangle vThumb = { vScrollbar.x, vThumbY, vScrollbar.width, vThumbH };
 
-            // Handle scrollbar dragging
+            // Handle scrollbar interaction. A press on the empty track jumps the
+            // view so the thumb re-centers under the cursor, then drags from there.
             static bool draggingH = false, draggingV = false;
             static Vector2 dragStartPos;
             static float dragStartViewStart, dragStartFreqViewStart;
 
-            if (IsMouseButtonPressed(MOUSE_LEFT_BUTTON) && CheckCollisionPointRec(GetMousePosition(), hScrollbar)) {
+            if (IsMouseButtonPressed(MOUSE_LEFT_BUTTON) && CheckCollisionPointRec(mouse, hScrollbar)) {
+                if (!CheckCollisionPointRec(mouse, hThumb) && hTravel > 0.0f) {
+                    float frac = Clamp((mouse.x - hScrollbar.x - hThumbW * 0.5f) / hTravel, 0.0f, 1.0f);
+                    app.view.start = frac * hDenom;
+                    app.view.end = app.view.start + hViewW;
+                    app.visibleTextureValid = false;
+                }
                 draggingH = true;
-                dragStartPos = GetMousePosition();
+                dragStartPos = mouse;
                 dragStartViewStart = app.view.start;
             }
-            if (IsMouseButtonPressed(MOUSE_LEFT_BUTTON) && CheckCollisionPointRec(GetMousePosition(), vScrollbar)) {
+            if (IsMouseButtonPressed(MOUSE_LEFT_BUTTON) && CheckCollisionPointRec(mouse, vScrollbar)) {
+                if (!CheckCollisionPointRec(mouse, vThumb) && vTravel > 0.0f) {
+                    float frac = Clamp((mouse.y - vScrollbar.y - vThumbH * 0.5f) / vTravel, 0.0f, 1.0f);
+                    app.view.freqEnd = 1.0f - frac * vDenom;
+                    app.view.freqStart = app.view.freqEnd - vViewH;
+                    app.visibleTextureValid = false;
+                }
                 draggingV = true;
-                dragStartPos = GetMousePosition();
+                dragStartPos = mouse;
                 dragStartFreqViewStart = app.view.freqStart;
             }
             if (draggingH && IsMouseButtonDown(MOUSE_LEFT_BUTTON)) {
-                float dx = (GetMousePosition().x - dragStartPos.x) / hScrollbar.width;
+                float dx = (hTravel > 0.0f) ? ((mouse.x - dragStartPos.x) / hTravel) * hDenom : 0.0f;
-                float viewWidth = app.view.end - app.view.start;
                 app.view.start = dragStartViewStart + dx;
-                app.view.end = app.view.start + viewWidth;
+                app.view.end = app.view.start + hViewW;
-                if (app.view.start < 0) { app.view.start = 0; app.view.end = viewWidth; }
+                if (app.view.start < 0) { app.view.start = 0; app.view.end = hViewW; }
-                if (app.view.end > 1) { app.view.end = 1; app.view.start = 1 - viewWidth; }
+                if (app.view.end > 1) { app.view.end = 1; app.view.start = 1 - hViewW; }
                 app.visibleTextureValid = false;
             }
             if (draggingV && IsMouseButtonDown(MOUSE_LEFT_BUTTON)) {
-                float dy = (GetMousePosition().y - dragStartPos.y) / vScrollbar.height;
+                float dy = (vTravel > 0.0f) ? ((mouse.y - dragStartPos.y) / vTravel) * vDenom : 0.0f;
-                float freqWidth = app.view.freqEnd - app.view.freqStart;
                 app.view.freqStart = dragStartFreqViewStart - dy;
-                app.view.freqEnd = app.view.freqStart + freqWidth;
+                app.view.freqEnd = app.view.freqStart + vViewH;
                 if (app.view.freqStart < 0) {
                     app.view.freqStart = 0;
-                    app.view.freqEnd = fminf(freqWidth, 1.0f);
+                    app.view.freqEnd = fminf(vViewH, 1.0f);
                 }
                 if (app.view.freqEnd > 1) {
                     app.view.freqEnd = 1;
-                    app.view.freqStart = fmaxf(1.0f - freqWidth, 0.0f);
+                    app.view.freqStart = fmaxf(1.0f - vViewH, 0.0f);
                 }
                 if (app.view.freqStart < 0) app.view.freqStart = 0;
                 if (app.view.freqEnd > 1) app.view.freqEnd = 1;
@@ -1642,6 +1665,18 @@ int main(int argc, char* argv[])
             }
             if (IsMouseButtonReleased(MOUSE_LEFT_BUTTON)) { draggingH = false; draggingV = false; }
 
+            // Draw tracks + thumbs after interaction so hover/active state and the
+            // jumped position are reflected this frame.
+            DrawRectangleRec(hScrollbar, trackColor);
+            DrawRectangleLinesEx(hScrollbar, 1.0f, trackBorder);
+            DrawRectangleRounded(hThumb, 0.5f, 4,
+                (draggingH || CheckCollisionPointRec(mouse, hThumb)) ? thumbHot : thumbColor);
+
+            DrawRectangleRec(vScrollbar, trackColor);
+            DrawRectangleLinesEx(vScrollbar, 1.0f, trackBorder);
+            DrawRectangleRounded(vThumb, 0.5f, 4,
+                (draggingV || CheckCollisionPointRec(mouse, vThumb)) ? thumbHot : thumbColor);
+
             if (app.showGrid) DrawSpectrogramGrid(viewBounds, 10, 8, Fade(GRAY, 0.3f));
             // Timeline lane sits above the spectrogram. Drawn before the
             // overlays so its hover/selection state is set for the same frame.