Add design doc: Reducing VisiblePosition usage in editing commands

Editing/visible_position/DESIGN.md

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+# Design: Reducing VisiblePosition Usage in Editing Commands
+
+---
+
+## Background
+
+`VisiblePosition` is Blink's canonicalized cursor position type - it answers *where the caret visually appears given current layout*. It belongs at exactly two points in the editing pipeline:
+
+- **Entry** - renderer hands a `VisibleSelection` to the command system.
+- **Exit** - command hands its result back to update the caret on screen.
+
+Inside `DoApply()`, commands work on DOM structure - raw `Position` questions, no layout needed. VP is used throughout command bodies as a type adapter because `IsEndOfParagraph`, `MoveParagraph`, etc. had no `Position` overloads when written. Every such call silently forces a full layout update, interleaved with DOM mutations.
+
+---
+
+## Problem Statement
+
+### User-visible symptoms
+- **Typing lag** - each keystroke triggers multiple full style+layout passes against a partially-mutated tree.
+- **Paste jank** - `ReplaceSelectionCommand` triggers dozens of layout passes for a single Ctrl+V on complex content.
+- **Slow indent/outdent** - `IndentOutdentCommand` loops per-paragraph; VP construction multiplies layout cost by paragraph count.
+- **Wrong-caret bugs** - stale VPs are silently used after DOM mutations (`IsValid()` is `DCHECK`-only in release). Caret lands in wrong position after paste, list insertion, or indent.
+- **Blocks future work** - every VP inside `DoApply()` is an implicit main-thread layout dependency, blocking batched undo, speculative editing, and off-thread IME.
+
+### Known bugs blocked on this work
+- [Hotlist 7675360](https://issues.chromium.org/hotlists/7675360) - 25-30 open issues: wrong caret after paste, bad selection after list insertion, undo/redo producing wrong selection, indent leaving caret in detached subtree.
+- Several WPT failures in `editing/` and `contenteditable/` where stale VP after mutation is the root cause.
+
+### Scale
+
+| Metric | Count |
+|--------|-------|
+| `VisiblePosition` type usage | ~391 across 18 files |
+| `CreateVisiblePosition()` calls | ~126 across 16 files |
+| `.DeepEquivalent()` calls (VP unwrapping) | ~288 across 18 files |
+
+Top offenders: `composite_edit_command.cc` (112), `replace_selection_command.cc` (98), `insert_list_command.cc` (91), `delete_selection_command.cc` (64), `indent_outdent_command.cc` (61).
+
+~110 of 126 `CreateVisiblePosition` calls (~87%) are unnecessary type adapters. Only ~16 sites are genuinely load-bearing.
+
+### After this work
+
+| Metric | Before | After |
+|--------|--------|-------|
+| `CreateVisiblePosition` calls in `commands/` | 126 | ~16 |
+| `VisiblePosition` type references in `commands/` | ~391 | ~66 |
+| Unnecessary `UpdateStyleAndLayout` per editing op | multiple | 0 |
+
+---
+
+### The Correct Abstraction Boundary
+
+```
+User Interaction
+    |
+    v
+[Boundary-IN]   VisibleSelection → SelectionInDOMTree   (renderer → command)
+    |
+    v
+[Command body]  Works entirely with Position / PositionWithAffinity / EphemeralRange
+    |
+    v
+[Boundary-OUT]  SelectionInDOMTree → SetEndingSelection(...)  (command → renderer)
+```
+
+`VisiblePosition` belongs only at the two boundary points. The problem is that commands create VPs deep inside `DoApply()`.
+
+---
+
+## Anti-Patterns to Eliminate
+
+Patterns are ordered by priority: highest VP count reduction first, then safety risk.
+
+### P1: VP parameter forces caller canonicalization
+
+`MoveParagraph` / `MoveParagraphWithClones` / `MoveParagraphs` in
+`composite_edit_command.h` take `const VisiblePosition&`. Every caller in
+`indent_outdent_command.cc`, `insert_list_command.cc`, `delete_selection_command.cc`,
+etc. must wrap raw positions into VPs before calling. Fixing the three method
+signatures (Step 3) removes ~80 VPs across all callers at once.
+
+### P2: `EndingVisibleSelection()` called mid-command for raw position access
+
+`EndingVisibleSelection()` unconditionally calls `UpdateStyleAndLayout` then wraps
+the result in a `VisibleSelection`. It appears in two unnecessary forms throughout
+`replace_selection_command.cc`, `composite_edit_command.cc`,
+`indent_outdent_command.cc`, `insert_list_command.cc`, and others:
+
+| Sub-pattern | Replacement |
+|---|---|
+| Raw position only | `EndingSelection().Start()` / `.End()` - no layout |
+| After explicit layout gate, position used for navigation | Keep `UpdateStyleAndLayout`; replace `EndingVisibleSelection().VisibleStart()` with `EndingSelection().Start()` and use `Position` overloads |
+| VP from `EndingVisibleSelection()` passed directly to navigation function | `NavigationFn(EndingSelection().Start(), rule)` - explicit gate already covers layout |
+
+`EndingSelection()` returns a `SelectionForUndoStep` with raw `Position` values that
+are already correct - no re-canonicalization needed.
+
+### P3: VP stored across a DOM mutation
+
+A `VisiblePosition` is captured before a DOM mutation and referenced after.
+`IsValid()` is `DCHECK`-only in release builds, so stale VP is silently used rather
+than caught. See crbug.com/648949. Low frequency but highest safety risk - silent
+wrong-caret corruption with no assertion to catch it.
+
+### P4: VP as a type requirement for navigation tests
+
+`IsEndOfParagraph` / `IsStartOfParagraph` have no `Position` overload today, so VP
+creation is forced purely to satisfy the type signature. High frequency across all
+command files; mechanical to fix once Step 1 overloads exist.
+
+### P5: Create-then-extract
+
+A VP is created from a `PositionWithAffinity` or raw `Position` and immediately
+passed to a navigation function. The VP serves only as a type adapter - the position
+was already correct before the wrap. Fixed mechanically alongside P4 sites.
+
+---
+
+## Proposed Approach: bottom-up foundation, top-down infrastructure
+
+1. **Add missing `Position`-based overloads** for navigation functions in `position_units` and `editing_commands_utilities` (bottom-up, purely additive).
+2. **Change `CompositeEditCommand` infrastructure signatures** to take `Position` instead of `VisiblePosition` (top-down, uses the new overloads internally).
+3. **Migrate command files** using the new overloads and the new infrastructure signatures, in ascending order of VP count.
+
+---
+
+## Step 1: Add Position-based Navigation Overloads
+
+**Nature:** Purely additive. No existing code is changed. Each overload is its own CL with tests. Zero regression risk.
+
+### New file: `position_units.h` / `position_units.cc`
+
+All `Position`-based navigation overloads live in a new dedicated pair. No
+`VisiblePosition` type appears anywhere in these files. The existing
+`Position`/`PositionWithAffinity` overloads currently in `visible_units.h` are
+relocated here in CL 1-A; the originals are removed. Callers that needed only those
+overloads switch to including `position_units.h`; files that still use VP-returning
+functions keep `visible_units.h`.
+
+### Gaps to fill
+
+New `Position` overloads to add to `position_units.h` / `position_units.cc`:
+
+**Paragraph boundary** (call algorithm templates in `visible_units_paragraph.cc`):
+`StartOfParagraph`, `EndOfParagraph`, `IsStartOfParagraph`, `IsEndOfParagraph`,
+`StartOfNextParagraph`, `InSameParagraph`.
+
+**Document boundary**: `EndOfDocument`, `IsStartOfDocument`, `IsEndOfDocument`.
+
+**Position traversal**: `PreviousPositionOf(const Position&, EditingBoundaryCrossingRule)`,
+`NextPositionOf(const Position&, EditingBoundaryCrossingRule)`. These bypass the
+`CreateVisiblePosition` wrap in the existing VP overloads by calling
+`PreviousVisuallyDistinctCandidate` / `NextVisuallyDistinctCandidate` directly. Do
+not change existing VP signatures.
+
+**Character query**: `CharacterAfter(const Position&)`.
+
+**Line boundary**: `IsStartOfLine` and `IsEndOfLine` taking `const PositionWithAffinity&`
+and `const Position&`. No `Position` overloads exist today; needed because
+`IsStartOfLine(CreateVisiblePosition(pos, affinity))` appears in
+`editing_commands_utilities.cc` with no layout-free replacement.
+
+**In `editing_commands_utilities.h` / `.cc`** (not `position_units` - depends on
+block-structure helpers private to that file): `StartOfBlock`, `EndOfBlock`,
+`IsStartOfBlock`, `IsEndOfBlock`, `EnclosingEmptyListItem`.
+
+### Already in `visible_units.h` - relocate to `position_units.h`
+
+`StartOfWordPosition`, `EndOfWordPosition`, `StartOfLine(PositionWithAffinity)`,
+`EndOfLine(PositionWithAffinity)`, `InSameLine(PositionWithAffinity, PositionWithAffinity)`,
+`StartOfDocument(Position)`.
+
+### Implementation strategy
+
+New overloads in `position_units.cc` call through to the existing private algorithm
+templates in the originating files (e.g. `StartOfParagraphAlgorithm<EditingStrategy>`
+in `visible_units_paragraph.cc`). Algorithm templates are not moved. For functions
+with no existing `Position`-typed algorithm, write the algorithm first and have both
+the VP overload and the new `Position` overload call into it.
+
+---
+
+## Step 2: Migrate Simple Commands
+
+Migrate the 5 lowest-VP command files. These validate that the new overloads are
+drop-in replacements against the WPT editing test suite.
+
+| File | VP refs | Notes |
+|------|---------|-------|
+| `style_commands.cc` | 1 | |
+| `undo_step.cc` | 2 | |
+| `editor_command.cc` | 6 | |
+| `insert_line_break_command.cc` | 6 | P3 and P5 examples |
+| `insert_text_command.cc` | 7 | |
+
+### Migration patterns
+
+| Before | After |
+|---|---|
+| `CreateVisiblePosition(pos).DeepEquivalent()` | `pos` - drop entirely; do not substitute `CanonicalPositionOf` |
+| `IsEndOfParagraph(CreateVisiblePosition(pos))` | `IsEndOfParagraph(pos)` |
+| `StartOfParagraph(vp).DeepEquivalent()` | `StartOfParagraph(pos)` |
+| `VP::FirstPositionInNode(*node).DeepEquivalent()` | `Position::FirstPositionInNode(*node)` |
+| `VP::LastPositionInNode(*node).DeepEquivalent()` | `Position::LastPositionInNode(*node)` |
+| `LineBreakExistsAtVisiblePosition(vp)` | `LineBreakExistsAtPosition(pos)` |
+| `VisiblePosition::BeforeNode(*node)` | `Position::BeforeNode(*node)` |
+| `EndingVisibleSelection().VisibleStart().DeepEquivalent()` | `EndingSelection().Start()` |
+
+---
+
+## Step 3: Infrastructure Signature Changes
+
+Change the five `CompositeEditCommand` protected methods from `const VisiblePosition&`
+to `const Position&`. Method bodies migrate simultaneously using the Step 1 overloads.
+
+- `MoveParagraph`, `MoveParagraphs` - three params each
+- `MoveParagraphWithClones` - two params
+- `CleanupAfterDeletion` - one param with default
+- `ReplaceCollapsibleWhitespaceWithNonBreakingSpaceIfNeeded` - one param
+
+All callers in `indent_outdent_command.cc`, `insert_list_command.cc`,
+`delete_selection_command.cc`, `apply_block_element_command.cc`,
+`format_block_command.cc` drop their `CreateVisiblePosition` wrappers and pass
+`Position` directly.
+
+---
+
+## Step 4: Heavy Command Migration
+
+With Step 1 overloads and Step 3 infrastructure in place, the heavy files fall into
+two categories:
+
+- **Category A** (fixed by Step 3): VP created solely to pass to `MoveParagraph` / `MoveParagraphWithClones` / `CleanupAfterDeletion`.
+- **Category B** (fixed by Step 1): VP created solely to call `IsEndOfParagraph`, `IsStartOfParagraph`, `StartOfParagraph`, `EndOfParagraph`, `StartOfBlock`, etc.
+
+| File | VP refs | Primary category |
+|------|---------|-----------------|
+| `insert_paragraph_separator_command.cc` | 19 | B |
+| `apply_block_element_command.cc` | 37 | B + A |
+| `indent_outdent_command.cc` | 61 | A |
+| `delete_selection_command.cc` | 64 | A + B |
+| `insert_list_command.cc` | 91 | A + B |
+| `replace_selection_command.cc` | 98 | B |
+
+`composite_edit_command.cc` (112 refs): Step 3 covers the method signatures. The
+remaining VP in `RebalanceWhitespaceOnTextSubstring`,
+`MoveParagraphContentsToNewBlockIfNecessary`, `BreakOutOfEmptyListItem`,
+`BreakOutOfEmptyMailBlockquotedParagraph` are addressed in Step 4.
+
+### Estimated VP reduction
+
+| Work item | Estimated VPs removed |
+|-----------|-----------------------|
+| Simple command migration (Step 2) | ~25 |
+| Infrastructure signature changes (Step 3) | ~80 |
+| Heavy command migration (Step 4) | ~220 |
+| **Total** | **~325 of ~391 (83%)** |
+
+---
+
+## What is Legitimately VP (Do Not Remove)
+
+1. **Entry boundary.** `CompositeEditCommand` constructor calling `ComputeVisibleSelectionInDOMTreeDeprecated()`.
+
+2. **Exit boundary.** `EndingVisibleSelection()` in `Apply()` for the `IsRichlyEditablePosition` check.
+
+3. **Visual-caret equivalence checks.** When the question is "did the caret actually move?", VP equivalence is the right tool - some positions are structurally distinct but render to the same caret. `MostForwardCaretPosition(x) == MostForwardCaretPosition(y)` is often a layout-free replacement, but each site requires individual confirmation.
+
+4. **`VisiblePosition::FirstPositionInNode` / `LastPositionInNode` as equality tests.** Replaceable with `Position::` variants only after per-site confirmation that positions are canonical.
+
+---
+
+## Testing Strategy
+
+**Per-CL:**
+- `blink_unittests --gtest_filter="*Edit*"` - all pass
+- `third_party/blink/web_tests/editing/` - no new failures
+- `third_party/blink/web_tests/editing/contenteditable/` - no new failures
+
+**Per-step gate:**
+- Full `blink_unittests` suite
+- `content_browsertests` editing subset
+- Verify `visible_position.h` include count in `commands/` is decreasing
+
+**Manual smoke test:** Basic typing, delete/backspace, paste, undo/redo, list creation, indent/outdent, Enter in list items, table editing.
+
+**Regression monitoring:** Watch CQ for editing test failures for 2 weeks after each step lands.
+
+---
+
+## Next Steps: Beyond Commands
+
+- `core/editing/selection_adjuster.cc` - 28 VP refs, primarily P4/P5
+- `core/editing/editing_utilities.cc` - ~12 VP refs in internal helpers
+- `core/editing/serializers/styled_markup_serializer.cc` - 7 VP refs, P4/P5
+- `core/editing/editor.cc` - ~5 VP refs in internal helpers
+
+Out of scope until Step 4 lands.
+