Fix #12739: keep the restored current time on map load, snap only when no time is set

[offtherailz]

Description

On map load the Timeline re-snaps the guide layer time and can overwrite a current time restored from the saved config. This change keeps the restored value and snaps only when there is no current time to preserve.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix

Issue

What is the current behavior?

#12739

When a map with a time layer is reloaded with a saved current time, the Timeline snaps that time to the nearest layer domain value on load. If the saved instant is not exactly a domain value, the current time (and the WMS TIME sent to the layer) changes to a different date. The result also depends on the timing of the asynchronous multidimensional describeDomains request, so it is not deterministic.

Root cause: on MAP_CONFIG_LOADED, epics/dimension.js updateLayerDimensionDataOnMapLoad restores the current time and then asynchronously loads the domains. Meanwhile epics/timeline.js selects the guide layer (updateTimelineDataOnMapLoad, or onUpdateLayerDimensionData when none is selected). Selecting the guide layer snaps the time: both initializeSelectLayer and selectLayer default to snap = true, so snapTimeGuideLayer runs setCurrentTime(nearestDomainValue) over the restored value.

What is the new behavior?

The saved value wins when it exists; snapping happens only when there is no restored current time. On load the guide layer is still selected, but without snapping when a current time was restored from the config. User-driven layer selection keeps snapping as before.

Proposed fix, in web/client/epics/timeline.js:

• updateTimelineDataOnMapLoad: initializeSelectLayer(selectedLayer, isEmpty(currentTime)).
• syncTimelineGuideLayer: selectLayer(firstTimeLayer, isEmpty(currentTimeSelector(state))).

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• No

Other useful information

Regression tests added in web/client/epics/__tests__/timeline-test.js:

• syncTimelineGuideLayer selects with snap = true when no current time is set, and with snap = false when a current time is already set.
• updateTimelineDataOnMapLoad selects with snap = false when a current time is restored, and with snap = true (plus AUTOSELECT) when none is.

How to reproduce: load a saved map with a WMS time layer (WMTS multidimensional extension) whose stored dimensionData.currentTime is not one of the layer domain values and whose timelineData.selectedLayer points to that layer. Before the fix the time snaps to a different value on load; after the fix the saved time is kept.

[offtherailz]

Successfully created backport PR for 2026.02.xx:

• [Backport 2026.02.xx] Fix #12739: keep the restored current time on map load, snap only when no time is set #12751

[dsuren1]

@ElenaGallo Kindly test it in DEV. Thanks!