Two 50% translucent layers stack into 75% opacity

• Single-layer area samples `#7fa8d8` while the overlap samples `#6c7ab0`, a brightness drop over 20%
• Overlap alpha computes to `0.5 + 0.5 * 0.5 = 0.75` rather than 0.5
• `getComputedStyle(map.getPane('overlayPane')).opacity` returns `0.8` while the path has `fill-opacity: 0.5`, giving 0.4

1. Read the pane's computed `opacity` in the Elements panel
2. Compare the path's `fill-opacity` with the layer's `opacity` option for double application
3. Sample single-layer and overlap pixels with a color picker and back-solve the alpha
4. Check for global CSS setting `opacity` on `.leaflet-overlay-pane svg`
5. Check Mapbox GL for `fill-opacity` multiplied by a layer-level opacity expression

Control transparency in one place with `fillOpacity` only (`fillOpacity: 0.35`), or back-solve the per-layer alpha for a target overlap: for 0.5 overlap, `2a - a*a = 0.5` gives a of about 0.293.

α_result = α_top + α_bottom × (1 - α_top)
α = 0.5 + 0.5 × 0.5 = 0.75

const pane = map.getPane('overlayPane');
console.log(getComputedStyle(pane).opacity);        // 0.8  <- 额外的乘数
console.log(getComputedStyle(pane.querySelector('path')).fillOpacity);  // 0.5

• Setting both `opacity` and `fillOpacity`, so the values multiply and the result is darker than either intent
• Raising `fillOpacity` on the lower layer to compensate, which darkens the non-overlapping area too