fix(dnd): tear down anchored drop overlay when a drag leaves mid-flight

[mathuo]

Description

While an HTML5 drag was still active, leaving an anchored drop target (e.g. a tab in a floating group) and moving to dead space — or to a target in a different overlay container — left the drop overlay visible until the drag ended.

Reproduction: a floating group with two tabs; drag one tab over the other (a drop overlay appears on the target tab's edge), then move the mouse off the floating group — the overlay stays.

Cause: the HTML5 backend intentionally does not clear the shared anchor container on dragleave — doing so would kill the overlay's slide between adjacent targets and churn on the spurious dragleaves HTML5 fires when crossing a target's own child elements. But never clearing means the overlay lingers whenever the drag leaves to somewhere that doesn't re-render into that container.

Fix: on dragleave the target now schedules a clear on the anchor container (a new optional DropTargetTargetModel.scheduleClear) instead of doing nothing. The next target that renders into the same container cancels it (via getElements), so the overlay still slides between adjacent targets; but if the drag leaves to dead space or another container, the deferred clear fires and the overlay is torn down. The pointer backend already clears on drag-leave, so this is HTML5-only.

Relationship to #1534

This is the root-cause fix and also covers the cross-container end-of-drag case addressed by the earlier dragend safety net in #1534 (releasing elsewhere first involves leaving the target, which now schedules the clear). The #1534 dragend listener is retained as belt-and-suspenders for the narrow case of a drag that ends with no clearing dragleave at all (e.g. an ESC-cancel where the browser skips dragleave).

Type of change

• Bug fix

Affected packages

• dockview-core

How to test

Steps above (floating group, two tabs, drag one over the other, then leave the group — overlay no longer lingers).

Automated (droptargetDeferredClear.spec.ts):

• leaving a target to dead space tears the overlay down;
• sliding to another target in the same container preserves the overlay (the scheduled clear is cancelled by the re-render);
• leaving a floating-container target for an in-place (other-container) target clears it purely from the leave — no dragend needed.

Checklist

• yarn test passes (1683 dockview-core tests)
• tsc --noEmit typecheck clean
• biome format / biome lint clean
• I have added or updated tests where applicable
• Breaking changes are documented (none — scheduleClear is an optional addition to an internal model interface)

🤖 Generated with Claude Code

https://claude.ai/code/session_01PjU3r4fnmpLUz7Bx1SjNvP

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
73.2% Coverage on New Code
0.0% Duplication on New Code

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