[WIP] Add layer groups to the layer system and compositing pipeline

[Copilot]

Coding agent has begun work on Add Layer Groups to the Layer System and Compositing Pipeline and will update this pull request as work progresses.

Original prompt

I would like to add "groups" to layers. Can you add that?
So basically layers can be organized in group. a group can be collapsed/exapnded, toggled visibility, changed namen, set alpha and blend modes, dragged to reorder, ...
When compositing. first compose all layers in the a group, and thread that as merged layer for the rest of the compositing.

TITLE: Add Layer Groups to the Layer System and Compositing Pipeline

USER INTENT:
The user wants to introduce “groups” into the layer system so layers can be organized hierarchically and manipulated as grouped units in both the UI and compositing logic.

TASK DESCRIPTION:
Implement support for layer groups with behavior similar to common image-editing tools. The user’s required capabilities include:

• Layers can be placed into a group.
• A group supports:
  • Collapse/expand in the layer panel
  • Visibility toggle
  • Rename
  • Alpha/opacity control
  • Blend mode selection
  • Drag-and-drop reordering
• Compositing behavior must change:
  • First, composite all layers within a group.
  • Then treat that result as a single merged layer in the parent/root compositing stack.

EXISTING:

• No implementation details, code changes, or file-level updates were provided in the conversation.
• The conversation currently contains only the high-level feature request and expected behavior.

PENDING:
Everything is pending implementation, including:

1. Data model changes

   • Add a group entity/type in the layer model (likely tree/hierarchy instead of flat list).
   • Store group properties: name, expanded/collapsed state, visibility, alpha, blend mode, children ordering.

2. Layer panel / UI behavior

   • Render groups and nested layers.
   • Support collapse/expand interactions.
   • Enable group-level controls (visibility, name, alpha, blend mode).
   • Enable drag-and-drop reordering for groups and items in/out of groups.

3. Compositor update

   • Recursive/stack-based compositing where each group is precomposited internally.
   • Apply group alpha/blend mode when inserting merged group result into parent stack.
   • Ensure visibility handling at both group and layer level.

4. Interaction rules / edge cases

   • Define behavior for hidden groups, empty groups, nested groups, locked states (if applicable), and clipping/masks (if applicable).
   • Clarify reorder constraints and drop targets.

5. Persistence / serialization

   • Save/load group hierarchy and properties.

6. Testing

   • Unit and integration tests for hierarchy operations and compositing correctness.
   • UI tests for expand/collapse, rename, drag/reorder, visibility, and blend/alpha effects.

CODE STATE:
No files were discussed or modified in the provided conversation.
No snippets or diffs are available yet.

RELEVANT CODE/DOCUMENTATION SNIPPETS:
No existing code or documentation excerpts were provided.
Only requirement-level statement from user:

• “layers can be organized in group”
• group supports “collapsed/expanded, toggled visibility, changed name, set alpha and blend modes, dragged to reorder”
• compositing rule: “first compose all layers in a group, and treat that as merged layer for the rest of compositing”

OTHER NOTES:

• The user’s final compositing requirement implies a hierarchical compositing graph (or recursive pass), not a simple flat layer loop.
• A robust implementation should likely support nested groups, even if not explicitly stated, since grouping semantics and precompose behavior naturally extend recursively.
• “thread that as merged layer” appears to mean “treat as merged/precomposed layer” in the parent composition order.

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