perf(editor): replace editor rendering pipeline with standalone viewport-windowed editor architecture

[loerei]

Summary

This PR replaces the editor rendering pipeline with a standalone viewport-windowed editor architecture while preserving the existing editing model and document format. The primary goal is to reduce layout work during typing in very large notes (>100k characters) without changing user-visible behavior.

It scope-bounds layout measurements and formatting span processing, reducing measured execution duration inside SimplenoteEditText.onDraw() at 100,000+ characters (2,500+ lines) from ~80.5ms down to ~29.8ms, while preserving native touch-drag selection, restoring scroll position persistence, and restoring native Android OverScroller fling inertia scrolling.

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Performance Demonstration

The attached video demonstrates typing responsiveness on a document containing 108,440 characters and 2,658 lines:

• First Half: Modified version using the Viewport-Windowed Single-EditText Engine (~29.8ms measured SimplenoteEditText.onDraw() execution duration).
• Second Half: Original baseline implementation using the ScrollView-Wrapped Full-Doc Editor (~80.5ms measured SimplenoteEditText.onDraw() execution duration).

SimplenotePerfDiffDemo.mp4

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Why

1. Root Cause: Unbounded Layout Measurement

In the legacy architecture, SimplenoteEditText was nested inside a NestedScrollView. Because ScrollView measures child views with MeasureSpec.UNSPECIFIED, Android was forced to measure, layout, and render all 100,000+ characters across the entire document on every single keypress, resulting in a baseline execution duration of ~80.5ms inside SimplenoteEditText.onDraw().

2. User Impact: Main Thread Stalls & Cumulative Frame Skips

While 80.5ms represents a single execution pass inside SimplenoteEditText.onDraw(), rapid typing compounds this delay on the Main UI Thread:

• VSYNC Frame Skips: On 120Hz (8.3ms budget) or 60Hz (16.6ms budget) displays, an 80.5ms execution inside SimplenoteEditText.onDraw() exceeds the frame deadline, skipping multiple VSYNC render cycles.
• Queued Event Stalls: When multiple keys are pressed in quick succession during an active layout pass, pending text modification events are processed sequentially on the Main Thread. Each subsequent pass incurs another ~80.5ms execution cost, accumulating input lag before rendering the updated text.
• String Allocations: Calling full-text toString() on 100,000 characters repeatedly allocates temporary String objects on every keypress.

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Implementation Details

1. Layout & View Architecture (fragment_note_editor.xml & SimplenoteEditText.java)

• Unwrapped NestedScrollView: Replaced root and nested NestedScrollView wrappers with a clean FrameLayout & LinearLayout hierarchy (layout_weight="1"). SimplenoteEditText now manages its own vertical scrollbar (android:scrollbars="vertical").
• Native Text View: Changed base class of SimplenoteEditText from AppCompatMultiAutoCompleteTextView to native MultiAutoCompleteTextView to avoid AppCompat compatibility layer overhead.
• Render Performance Stack:
  • Disabled font padding (setIncludeFontPadding(false)) and elegant text height (setElegantTextHeight(false)).
  • Enforced Layout.BREAK_STRATEGY_SIMPLE and Layout.HYPHENATION_FREQUENCY_NONE (Android M+).
  • Preserved standard keyboard suggestions and autocorrect (textAutoCorrect), relying on standard window-level hardware acceleration.

2. Local Cursor Windowing & Incremental Scanning (SimplenoteEditText.java, NoteEditorFragment.java, & AutoBullet.java)

• Cursor Windowing $O(1)$ Optimization (enoughToFilter): Replaced full-text toString().substring() with a localized subSequence window inspecting max 200 characters around the cursor.
• Incremental Checklist Scanning: Overloaded processChecklists(int start, int count) to scan and apply checkbox spans only within the paragraph bounds of the active edit window instead of the whole file.
• Targeted Title Formatting (setTitleSpan): Restricted MetricAffectingSpan search to the title line range ([0, titleEndPosition + 1]).
• AutoBullet Early Exit: Added instant line-break check editable.charAt(newCursorPosition - 1) != '\n' to bypass toString() allocations on standard typing.

3. Scroll Position Persistence & Native Fling Inertia (SimplenoteMovementMethod.java & SimplenoteEditText.java)

• Scroll Position Persistence: Attached OnScrollChangeListener directly on SimplenoteEditText to preserve scroll position state across navigation and note opens.
• Touch Event Delegation: Updated SimplenoteMovementMethod.onTouchEvent() to return super.onTouchEvent(...), restoring drag-selection and cursor placement.
• Restored Native Fling Behavior: Integrated OverScroller and VelocityTracker in SimplenoteEditText.onTouchEvent(), restoring native Android fling deceleration and preserving standard platform scrolling dynamics upon touch release (ACTION_UP).

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Verification & Performance Benchmarks

1. Automated & Compilation Verification

• Ran .\gradlew assembleDebug $\rightarrow$ BUILD SUCCESSFUL.
• Ran .\gradlew testDebugUnitTest $\rightarrow$ BUILD SUCCESSFUL (All unit tests pass).
• Installed and verified on Samsung Galaxy S24 Ultra (SM-S928B).

2. Empirical Performance Metrics (108,440 characters / 2,658 lines)

• SimplenoteEditText.onDraw() Duration: Measured directly inside SimplenoteEditText.onDraw(), execution duration dropped from ~80.5ms (legacy trunk) down to ~29.8ms at 108,440 characters.
• Fling Inertia Scrolling: Deceleration matches native Android ScrollView feel.
• Checklist Toggling: Instant UI update without text watcher callback loops.

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Files Changed

Core Editor Engine

• Simplenote/src/main/java/com/automattic/simplenote/widgets/SimplenoteEditText.java: Implemented hardware render stack, OverScroller fling inertia, local cursor windowing, and incremental checklist processing.
• Simplenote/src/main/java/com/automattic/simplenote/utils/SimplenoteMovementMethod.java: Delegated touch events back to super.onTouchEvent for touch selection.

Fragment & Utility Optimizations

• Simplenote/src/main/java/com/automattic/simplenote/NoteEditorFragment.java: Attached scroll listener on SimplenoteEditText to persist scroll position, scoped setTitleSpan to title paragraph, and enabled incremental checklist processing.
• Simplenote/src/main/java/com/automattic/simplenote/utils/AutoBullet.java: Added early exit check for non-newline typing.

Layout & Build System

• Simplenote/src/main/res/layout/fragment_note_editor.xml: Removed NestedScrollView wrappers and configured standalone vertical scrollbar layout with textAutoCorrect.
• Simplenote/build.gradle: Configured debug build string resource value.

[loerei]

Hi team! Long-time user here. 👋

This is my first contribution to the project. I started looking into this after noticing typing lag once my personal diary grew to around 100k characters, which led me to explore a viewport-windowed approach to reduce layout work during editing.

I've verified the changes on my device, ran the existing unit test suite successfully, and included performance measurements and a demo video in the PR description. I'm happy to iterate on the implementation based on your feedback.