Fix inline editing second-edit stale value (#948)

[radimvaculik]

Problem

When a column uses setEditableValueCallback() together with inline editing, the first edit works correctly, but a second edit of the same cell shows the value from before the first edit.

Reported in #948.

Root cause

On render, the cell stores the raw editable value in the data-datagrid-editable-value attribute, while innerHTML holds the rendered representation. The client reads valueToEdit from that attribute.

After a successful edit, the client updates only innerHTML (to _datagrid_editable_new_value), but never updates the data-datagrid-editable-value attribute. So the next edit reads the stale value captured on the initial page load.

Fix

After a successful inline edit, sync data-datagrid-editable-value with the submitted input value, so subsequent edits start from the current value. Applies to both text/textarea and select inputs.

Purely client-side (assets/plugins/features/editable.ts); no server round-trip needed since the submitted raw value is already available on the client.

Closes #948

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.12%. Comparing base (ab5506d) to head (ea4b484).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1297   +/-   ##
=======================================
  Coverage   49.12%   49.12%           
=======================================
  Files          63       63           
  Lines        2974     2974           
=======================================
  Hits         1461     1461           
  Misses       1513     1513           

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