fix(html-layer): keep HTML overlays aligned when the viewer container has an offset

[FenjuFu]

What

Fixes #212 — HtmlLayer overlays drift vertically when the viewer container has an offset in the page (a container with a fixed height, or nested in an iframe). The issue does not occur in v2.x.

Root cause

HtmlLayer positions its DOM overlays from Cesium.SceneTransforms.worldToWindowCoordinates(), which already returns canvas-relative pixels. It then adds viewer.getOffset() — the container's getBoundingClientRect().left/top, i.e. its viewport position:

const offset = this._viewer.getOffset()
// ...
windowCoord.x += offset.x
windowCoord.y += offset.y

The overlays are position: absolute .div-icon elements inside .viewer-layers. When the container fills the screen at viewport (0,0), getOffset() is ~{0,0} and the extra term is invisible. Once the container sits lower in the page (e.g. height: 95px), rect.top grows and every overlay is pushed down by the container offset — the reported symptom. .viewer-layers also has no positioning of its own, so the overlays lacked a canvas-aligned positioned ancestor and the getOffset() term was only compensating by coincidence at the origin.

Change

• HtmlLayer.js: drop the getOffset() addition; the window coordinates are already canvas-relative.
• viewer.scss: give .viewer-layers an explicit box that overlaps the canvas so absolutely positioned overlays resolve against a canvas-aligned ancestor regardless of the outer container. pointer-events is disabled on the pane and re-enabled on its children so map interaction is preserved.

Together these make HTML overlays independent of where the viewer container sits in the page (fixed height, nested, iframe), which is the expected behaviour described in the issue.

Verification note

This was derived from source analysis and restores the pre-v5 behaviour. I could not run a full local build to verify in a browser, so please double-check the pointer-events change against the other panes mounted on layerContainer (WindLayer, HeatMapLayer, ChartLayer) to confirm their interaction is unaffected. Happy to adjust.

[FenjuFu]

Verified locally against a dc-sdk v5.0.0 build.

Repro: the viewer container is placed below a 200px header, so container.getBoundingClientRect().top ≈ 200. Three DivIcons and three Cesium-rendered Points are added at identical coordinates; the in-canvas points are the ground truth.

Before (current dev): .viewer-layers is position: static, and every DivIcon renders +200px below its point — exactly the container's page offset.

point (viewport x,y)	div-icon center	ΔY
408, 556	408, 756	+200
450, 519	450, 719	+200
492, 581	492, 781	+200

After (this PR): the offset addition is gone and .viewer-layers is the canvas-aligned box, so every DivIcon lands on its point — ΔY = 0 at all three, with the container still 200px down the page.

point (viewport x,y)	div-icon center	ΔY
213, 707	213, 707	0
247, 673	247, 673	0
281, 732	281, 732	0

pointer-events: the change follows the standard overlay-pane pattern (pane pointer-events: none, children auto) so the pane doesn't intercept map interaction while div-icons stay clickable. I did not specifically exercise the other panes mounted on layerContainer (WindLayer / HeatMapLayer / ChartLayer), so a maintainer check there is still welcome.