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Add comprehensive CWP pattern analysis and recommendations
Co-authored-by: tig <585482+tig@users.noreply.github.com>
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docs/analysis/cwp_analysis_report.md
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docs/analysis/cwp_analysis_report.md
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# Cancellable Work Pattern (CWP) Analysis Report
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## Executive Summary
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This report analyzes all instances of the Cancellable Work Pattern (CWP) in the Terminal.Gui codebase to assess the potential impact of reversing the calling order from:
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- **Current**: Virtual method first → Event second
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- **Proposed**: Event first → Virtual method second
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## Background
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The CWP pattern currently calls the virtual method (`OnXxx`) before invoking the event (`Xxx`). This gives inherited code (via override) priority over external code (via event subscription). Issue #3714 raises concerns about this order, particularly for mouse events where external code cannot prevent events from reaching overridden methods in views.
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## Analysis Methodology
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1. Identified all `protected virtual bool OnXxx` methods in Terminal.Gui
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2. Located their corresponding event invocations
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3. Analyzed the calling context and dependencies
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4. Assessed impact of order reversal
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## Detailed Analysis
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### 1. Mouse Events
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#### 1.1 OnMouseEvent / MouseEvent
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**File**: `Terminal.Gui/ViewBase/View.Mouse.cs:348`
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**Calling Context**:
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```csharp
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public bool RaiseMouseEvent (MouseEventArgs mouseEvent)
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{
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if (OnMouseEvent (mouseEvent) || mouseEvent.Handled)
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{
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return true;
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}
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MouseEvent?.Invoke (this, mouseEvent);
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return mouseEvent.Handled;
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}
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```
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**Current Order**: `OnMouseEvent` → `MouseEvent`
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**Impact Analysis**:
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- **HIGH IMPACT**: This is the primary mouse event handler
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- **Problem**: Views like `Slider` that override `OnMouseEvent` cannot be prevented from handling mouse events by external code
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- **Dependencies**: Many views override `OnMouseEvent` expecting priority
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- **Reversing Order Would**:
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- Allow external code to cancel before view's override processes
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- Break existing assumptions in views that expect first access
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- Require review of all `OnMouseEvent` overrides
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#### 1.2 OnMouseClick / MouseClick
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**File**: `Terminal.Gui/ViewBase/View.Mouse.cs:515`
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**Calling Context**:
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```csharp
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protected bool RaiseMouseClickEvent (MouseEventArgs args)
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{
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if (OnMouseClick (args) || args.Handled)
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{
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return args.Handled;
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}
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MouseClick?.Invoke (this, args);
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// ...
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}
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```
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**Current Order**: `OnMouseClick` → `MouseClick`
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**Impact Analysis**:
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- **HIGH IMPACT**: Same issue as `OnMouseEvent`
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- **Problem**: External code cannot prevent clicks from reaching view's override
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- **Reversing Order Would**:
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- Enable external mouse click prevention
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- Consistent with `OnMouseEvent` changes
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- Requires coordination with all click handlers
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#### 1.3 OnMouseEnter / MouseEnter
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**File**: `Terminal.Gui/ViewBase/View.Mouse.cs:125`
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**Calling Context**:
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```csharp
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internal bool? NewMouseEnterEvent (CancelEventArgs eventArgs)
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{
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if (OnMouseEnter (eventArgs))
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{
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return true;
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}
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MouseEnter?.Invoke (this, eventArgs);
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// ...
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}
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```
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**Current Order**: `OnMouseEnter` → `MouseEnter`
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**Impact Analysis**:
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- **MEDIUM IMPACT**: Less commonly overridden
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- **Reversing Order Would**:
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- Allow external code to prevent enter notifications
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- Minimal breaking changes expected
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#### 1.4 OnMouseWheel / MouseWheel
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**File**: `Terminal.Gui/ViewBase/View.Mouse.cs:610`
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**Calling Context**:
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```csharp
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protected bool RaiseMouseWheelEvent (MouseEventArgs args)
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{
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if (OnMouseWheel (args) || args.Handled)
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{
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return args.Handled;
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}
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MouseWheel?.Invoke (this, args);
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// ...
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}
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```
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**Current Order**: `OnMouseWheel` → `MouseWheel`
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**Impact Analysis**:
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- **MEDIUM IMPACT**: Wheel handling is specific
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- **Reversing Order Would**:
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- Enable external scroll prevention
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- Useful for modal dialogs or locked views
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### 2. Keyboard Events
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#### 2.1 OnKeyDown / KeyDown
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**File**: `Terminal.Gui/ViewBase/View.Keyboard.cs:367`
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**Calling Context**:
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```csharp
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public bool NewKeyDownEvent (Key k)
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{
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if (OnKeyDown (k) || k.Handled)
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{
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return true;
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}
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KeyDown?.Invoke (this, k);
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return k.Handled;
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}
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```
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**Current Order**: `OnKeyDown` → `KeyDown`
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**Impact Analysis**:
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- **HIGH IMPACT**: Core keyboard input handling
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- **Dependencies**: Many views override for input processing
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- **Reversing Order Would**:
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- Allow external key interception before view processing
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- Could break views expecting first access to keys
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- Major behavioral change for input handling
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#### 2.2 OnKeyDownNotHandled / KeyDownNotHandled
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**File**: `Terminal.Gui/ViewBase/View.Keyboard.cs:399`
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**Current Order**: `OnKeyDownNotHandled` → `KeyDownNotHandled`
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**Impact Analysis**:
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- **LOW IMPACT**: Secondary handler for unhandled keys
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- **Reversing Order Would**:
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- Minor change, already handles unhandled keys
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- Low risk
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#### 2.3 OnKeyUp / KeyUp
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**File**: `Terminal.Gui/ViewBase/View.Keyboard.cs` (implementation in NewKeyUpEvent)
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**Current Order**: `OnKeyUp` → `KeyUp`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Less commonly used than KeyDown
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- **Reversing Order Would**:
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- Enable external key-up prevention
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- Lower risk than KeyDown
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### 3. Command Events
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#### 3.1 OnAccepting / Accepting
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**File**: `Terminal.Gui/ViewBase/View.Command.cs:190`
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**Impact Analysis**:
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- **MEDIUM-HIGH IMPACT**: Core command pattern
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- **Used by**: CheckBox, RadioGroup, Button, etc.
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- **Reversing Order Would**:
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- Allow external cancellation before view logic
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- May break views expecting to set state first
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#### 3.2 OnSelecting / Selecting
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**File**: `Terminal.Gui/ViewBase/View.Command.cs:245`
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**Impact Analysis**:
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- **MEDIUM IMPACT**: Selection behavior
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- **Reversing Order Would**:
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- Enable external selection control
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- Useful for validation scenarios
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#### 3.3 OnHandlingHotKey / HandlingHotKey
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**File**: `Terminal.Gui/ViewBase/View.Command.cs:291`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: HotKey processing
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- **Reversing Order Would**:
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- Allow hotkey override by external code
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- Could be useful for dynamic hotkey management
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#### 3.4 OnCommandNotBound / CommandNotBound
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**File**: `Terminal.Gui/ViewBase/View.Command.cs:93`
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**Impact Analysis**:
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- **LOW IMPACT**: Fallback for unmapped commands
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- **Reversing Order Would**:
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- Minor impact, handles unbound commands
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### 4. Drawing Events
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#### 4.1 Drawing Pipeline Events
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**Files**: `Terminal.Gui/ViewBase/View.Drawing.cs`
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- `OnDrawingAdornments` / `DrawingAdornments`
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- `OnClearingViewport` / `ClearingViewport`
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- `OnDrawingText` / `DrawingText`
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- `OnDrawingContent` / `DrawingContent`
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- `OnDrawingSubViews` / `DrawingSubViews`
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- `OnRenderingLineCanvas` / `RenderingLineCanvas`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Rendering customization
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- **Reversing Order Would**:
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- Allow external drawing interception
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- Useful for debugging/tracing
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- Lower risk as drawing is less state-dependent
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### 5. Navigation Events
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#### 5.1 OnAdvancingFocus / AdvancingFocus
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**File**: `Terminal.Gui/ViewBase/View.Navigation.cs:208`
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**Impact Analysis**:
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- **MEDIUM-HIGH IMPACT**: Focus management
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- **Reversing Order Would**:
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- Allow external focus control
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- Could break focus flow expectations
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#### 5.2 OnHasFocusChanging / HasFocusChanging
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**File**: `Terminal.Gui/ViewBase/View.Navigation.cs:717`
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**Impact Analysis**:
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- **MEDIUM-HIGH IMPACT**: Focus state management
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- **Reversing Order Would**:
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- Enable external focus validation
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- Useful for preventing focus changes
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### 6. Property Change Events
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#### 6.1 OnSchemeNameChanging / SchemeNameChanging
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**File**: `Terminal.Gui/ViewBase/View.Drawing.Scheme.cs:51`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Scheme change validation
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- **Uses**: `CWPPropertyHelper`
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- **Reversing Order Would**:
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- Allow external scheme validation
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- Low risk
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#### 6.2 OnGettingScheme / GettingScheme
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**File**: `Terminal.Gui/ViewBase/View.Drawing.Scheme.cs:170`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Scheme resolution
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- **Reversing Order Would**:
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- Allow external scheme override
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- Useful for theming
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#### 6.3 OnSettingScheme / SettingScheme
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**File**: `Terminal.Gui/ViewBase/View.Drawing.Scheme.cs:241`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Scheme application
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- **Reversing Order Would**:
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- Enable external scheme interception
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- Low risk
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#### 6.4 OnVisibleChanging / VisibleChanging
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**File**: `Terminal.Gui/ViewBase/View.cs:382`
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**Impact Analysis**:
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- **MEDIUM IMPACT**: Visibility state management
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- **Reversing Order Would**:
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- Allow external visibility control
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- Could affect layout calculations
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#### 6.5 OnOrientationChanging (commented out)
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**File**: `Terminal.Gui/ViewBase/Orientation/OrientationHelper.cs:166`
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**Impact Analysis**:
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- **N/A**: Currently commented out
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- **Note**: OrientationHelper uses callback pattern
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### 7. View-Specific Events
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#### 7.1 OnCellActivated (TableView)
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**File**: `Terminal.Gui/Views/TableView/TableView.cs:1277`
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**Impact Analysis**:
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- **LOW IMPACT**: TableView-specific
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- **Reversing Order Would**:
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- Allow external cell activation control
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- Isolated to TableView
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#### 7.2 OnPositionChanging (ScrollBar, ScrollSlider)
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**Files**:
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- `Terminal.Gui/Views/ScrollBar/ScrollBar.cs:375`
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- `Terminal.Gui/Views/ScrollBar/ScrollSlider.cs:245`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: Scrollbar-specific
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- **Reversing Order Would**:
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- Enable external scroll validation
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- Useful for scroll constraints
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#### 7.3 OnCheckedStateChanging (CheckBox)
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**File**: `Terminal.Gui/Views/CheckBox.cs:187`
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**Impact Analysis**:
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- **LOW-MEDIUM IMPACT**: CheckBox state management
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- **Reversing Order Would**:
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- Allow external state validation
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- Useful for validation logic
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### 8. Other Events
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#### 8.1 OnBorderStyleChanged
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**File**: `Terminal.Gui/ViewBase/View.Adornments.cs:162`
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**Impact Analysis**:
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- **LOW IMPACT**: Border styling
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- **Reversing Order Would**:
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- Enable external border control
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- Minimal risk
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#### 8.2 OnMouseIsHeldDownTick (MouseHeldDown)
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**File**: `Terminal.Gui/ViewBase/MouseHeldDown.cs:71`
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**Impact Analysis**:
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- **LOW IMPACT**: Continuous mouse press
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- **Reversing Order Would**:
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- Allow external hold-down control
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- Minimal risk
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#### 8.3 OnGettingAttributeForRole
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**File**: `Terminal.Gui/ViewBase/View.Drawing.Attribute.cs:78`
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**Impact Analysis**:
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- **LOW IMPACT**: Attribute resolution
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- **Reversing Order Would**:
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- Enable external attribute override
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- Useful for accessibility
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## Summary Statistics
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**Total CWP Implementations Found**: 33
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**By Impact Level**:
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- **HIGH IMPACT**: 3 (Mouse/Keyboard core events)
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- **MEDIUM-HIGH IMPACT**: 4 (Commands, Navigation)
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- **MEDIUM IMPACT**: 8 (Various UI events)
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- **LOW-MEDIUM IMPACT**: 10 (Property changes, view-specific)
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- **LOW IMPACT**: 8 (Specialized/rare events)
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## Code Dependencies on Current Order
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### Tests That Validate Order
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Located in `Tests/UnitTestsParallelizable/View/Orientation/OrientationTests.cs`:
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```csharp
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[Fact]
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public void OrientationChanging_VirtualMethodCalledBeforeEvent()
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{
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// Test explicitly validates virtual method is called before event
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}
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[Fact]
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public void OrientationChanged_VirtualMethodCalledBeforeEvent()
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{
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// Test explicitly validates virtual method is called before event
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}
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```
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**Impact**: These tests will FAIL if order is reversed - they explicitly test current behavior.
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### Views With Override Dependencies
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Based on code search, many views override these methods and may depend on being called first:
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- `Slider.OnMouseEvent` - Core issue from #3714
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- Various views override `OnKeyDown` for input handling
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- Command handlers in Button, CheckBox, RadioGroup
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- Custom drawing in various views
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## Recommendations
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### Option 1: Reverse Order Globally
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**Pros**:
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- Solves #3714 completely
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- Consistent pattern across all events
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- External code gets priority (looser coupling)
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**Cons**:
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- MAJOR BREAKING CHANGE
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- Requires updating all views that override CWP methods
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- Extensive testing required
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- May break user code
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**Effort**: HIGH (4-6 weeks)
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**Risk**: HIGH
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### Option 2: Add "Before" Events
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**Pros**:
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- Non-breaking
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- Explicit control for when needed
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- Gradual migration path
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**Cons**:
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- More API surface
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- Complexity in naming/documentation
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- Two patterns coexist
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**Effort**: MEDIUM (2-3 weeks)
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**Risk**: LOW
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### Option 3: Add `IgnoreMouse` Property
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**Pros**:
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- Minimal change
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- Solves immediate #3714 issue
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- No breaking changes
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**Cons**:
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- Band-aid solution
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- Doesn't address root cause
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- Only solves mouse issue
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**Effort**: LOW (1 week)
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**Risk**: VERY LOW
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### Option 4: Reverse Order for Mouse Events Only
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**Pros**:
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- Solves #3714
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- Limited scope reduces risk
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- Mouse is primary concern
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**Cons**:
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- Inconsistent pattern
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- Still breaking for mouse overrides
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- Confusion with different orders
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**Effort**: MEDIUM (2 weeks)
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**Risk**: MEDIUM
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## Conclusion
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The analysis reveals that reversing CWP order globally would be a **significant breaking change** affecting 33+ event pairs across the codebase. The **highest impact** would be on:
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1. **Mouse events** (OnMouseEvent, OnMouseClick) - direct issue #3714
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2. **Keyboard events** (OnKeyDown) - core input handling
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3. **Command events** (OnAccepting, OnSelecting) - state management
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4. **Navigation events** (OnAdvancingFocus, OnHasFocusChanging) - focus flow
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**Recommended Approach**:
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- **Short-term**: Option 3 (IgnoreMouse property) or Option 2 (Before events for mouse only)
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- **Long-term**: Consider Option 2 with gradual migration, or accept current design as intentional
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The current design prioritizes **inheritance over composition**, which may be appropriate for a UI framework where tight coupling through inheritance is common. However, it limits external control, which is the root cause of #3714.
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## Next Steps
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1. Review this analysis with maintainers
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2. Decide on approach based on project priorities
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3. If proceeding with order reversal:
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- Create comprehensive test plan
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- Update all affected views
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- Update documentation
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- Provide migration guide
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4. If proceeding with alternative:
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- Implement chosen solution
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- Update documentation with rationale
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- Add examples for working around current limitation
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