Improve Split View handling

[hybridmachine]

The current Split View is not reliable. Unsplitting will move items to the remaining view but the text area displays as blank and the app becomes unusable. Review the Split View implementation and improve it, making it more reliable so that both split and unspilt work correctly no matter how many times they are selected.

[hybridmachine]

Implementation Plan: Fix Split View Handling

Root Cause Analysis

Three distinct bugs compound to make split view unreliable:

Bug 1 — Documents lost on unsplit: doUnsplit() at split_view.mm:346 calls ctx().documents2.clear(), silently discarding all documents from view 2. Any documents moved/cloned/opened in the second view are permanently lost.

Bug 2 — Blank editor after unsplit: The function reparents editorContainer (removes from NSSplitView, adds to contentView) but never refreshes the Scintilla display. Missing: (a) restoreViewToScintilla() to reload content into the editor, (b) ScintillaBridge_focus() to set first responder, (c) deferred display invalidation for Cocoa layout timing, (d) save of main view state before reparenting.

Bug 3 — Missing safety guard in doSplit: isSplit = true is set outside the if (ctx().scintillaView2) block, so a failed ScintillaBridge_createView leaves the app in an inconsistent half-split state with no second editor but the flag claiming there is one.

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Implementation Steps

Step 1: Add migrateTabToView() helper to document_manager.mm

Add a data-only function that appends a DocumentData to a target view's document vector and tab bar, without touching the Scintilla view. This is needed because during unsplit, scintillaView2 is already destroyed before migration runs.

void migrateTabToView(int viewIndex, const DocumentData& doc)

• Pushes doc onto the target view's documents vector
• Inserts a tab via TCM_INSERTITEMW
• Updates modified indicator
• Does NOT change active tab or call SCI_SETTEXT

Step 2: Rewrite doUnsplit() in split_view.mm

Key changes to the unsplit flow:

1. Save both views' state before destroying anything (currently only saves view 2)
2. Snapshot documents2 before clearing, for migration
3. After reparenting editorContainer:
   • Call restoreViewToScintilla() to reload content into the main Scintilla (fixes blank editor)
   • Call applyLanguageToView() to restore syntax highlighting
4. Migrate documents from view 2 to view 0:
   • Skip files already open in view 0 (dedup by filePath)
   • Skip empty untitled unmodified documents
   • Use migrateTabToView() for the rest
5. After UI refresh:
   • Call ScintillaBridge_focus() to set keyboard focus
   • Update window title, status bar, file switcher
   • Schedule deferred setNeedsDisplay:YES via dispatch_async to handle Cocoa layout timing

Step 3: Add safety guard to doSplit() in split_view.mm

• Save original editorContainer.frame before modification
• If ScintillaBridge_createView returns null, roll back: restore editorContainer to contentView, nil the split view components, return early
• Move isSplit = true inside the success path

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Verification Plan

1. Basic cycle: Open file → split → unsplit → repeat 5+ times — editor should never go blank
2. Document migration: Split → move tab to view 2 → unsplit — tab preserved in view 0
3. Clone dedup: Split → clone to view 2 → unsplit — no duplicate in view 0
4. Focus edge case: Split → click in view 2 → unsplit — view 0 displays correctly
5. Modified state: Split → edit in view 2 → unsplit — modified indicator preserved
6. Session restore: Split → quit → relaunch — split state restores correctly

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Files Changed

File	Change
macos/platform/split_view.mm	Rewrite doUnsplit(), add safety guard to doSplit()
macos/platform/document_manager.mm	Add migrateTabToView() helper
macos/platform/document_manager.h	Declare migrateTabToView()

[hybridmachine]

Solid plan overall — it nails the two obvious correctness bugs in split_view.mm (documents2.clear() on unsplit, and isSplit = true outside the scintillaView2 success path).

My main critique is the merge step:

1. Deduping by filePath is unsafe with the current implementation. doCloneToOtherView() creates a second DocumentData copy, not a shared buffer, so a clone can diverge from the original. If unsplit drops one side just because the paths match, it can silently discard edits. I’d either preserve both tabs on merge, or first change clone semantics to a real shared-buffer model.
2. The plan should explicitly preserve the active document. A helper that only appends tabs to view 0 still leaves the user on whatever was active in view 0 before unsplit. If the user was editing view 2, unsplit should select that migrated tab, then refresh title/status/document-edited/file-switcher state from that tab.
3. If session restore is part of verification, it probably needs code changes too: restoreSession() rebuilds view 2 via openFileAtPath(), and that path dedupes files across both views. Same-file tabs/clones won’t round-trip unless that restore path is handled separately.

Smaller note: if any view-2 docs are intentionally dropped during merge, invalidate their function-list cache entries instead of just clearing the vector.

[hybridmachine]

Amended Plan (addressing review feedback)

Changes from v1

1. No dedup by filePath. Clones create independent DocumentData copies that can diverge. Dropping one side on unsplit would silently discard edits. The amended plan migrates all view-2 tabs to view 0 unconditionally (only skipping the empty untitled placeholder). The user can manually close duplicates they don't need.

2. Preserve active document. If the user was focused on view 2 (activeView == 1), the plan now tracks which migrated tab corresponds to the view-2 active document, and calls switchToTabInView(0, migratedIdx) so the user lands on that document after unsplit.

3. Function list cache invalidation. Any docsToMigrate entries that are skipped during migration get their functionListDocumentId invalidated via invalidateFunctionListCacheForDocument() before clearing the vector.

4. Session restore for clones is out of scope. restoreSession() rebuilds view 2 via openFileAtPath() which dedupes across views. Same-file clones won't round-trip. This is a pre-existing limitation, not introduced by this fix — tracked separately.

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Revised doUnsplit() Flow

 1. Guard: if (!isSplit || !splitView) return
 2. Save view 2 state to documents2
 3. NEW: Save view 0 state to documents
 4. NEW: Capture focus: wasActiveInView2 = (activeView == 1)
 5. NEW: Snapshot docsToMigrate = documents2
 6. Handle incremental search bar removal (existing)
 7. Destroy scintillaView2 (existing)
 8. Destroy tabHwnd2 (existing)
 9. Reparent editorContainer to contentView (existing)
10. ScintillaBridge_resizeToFit (existing)
11. NEW: Migrate ALL view-2 docs to view 0 (no filePath dedup).
        Skip only empty untitled unmodified placeholder docs.
        Track where view2's active tab lands in view 0.
12. NEW: Invalidate function list cache for skipped docs
13. Clear documents2, reset state flags
14. NEW: If wasActiveInView2, switchToTabInView(0, migratedIdx)
15. restoreViewToScintilla + applyLanguageToView
16. Layout + UI refresh (existing)
17. NEW: ScintillaBridge_focus, window title, status bar, file switcher
18. NEW: Deferred dispatch_async for display refresh

Files Changed (unchanged from v1)

File	Change
macos/platform/split_view.mm	Rewrite doUnsplit(), add safety guard to doSplit()
macos/platform/document_manager.mm	Add migrateTabToView() helper
macos/platform/document_manager.h	Declare migrateTabToView()

Updated Verification

• Clone preservation: Split → clone to view 2 → edit the clone → unsplit → both copies preserved in view 0 with independent content
• Active document follows focus: Split → click in view 2 → edit → unsplit → that document is now active in view 0
• All other test cases from v1 remain the same