Fixes gui-cs#4677 - Refactors DimAuto for less coupling and improves performance (gui-cs#4678)

* Phase 5: Add IsFixed and RequiresTargetLayout categorization properties

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Further simplify DimAuto.Calculate using IsFixed property

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Refactor for-loops to foreach and clarify DimFill logic

Refactored multiple for-loops iterating over view lists to use foreach loops for improved readability and reduced boilerplate. Removed unused variables such as viewsNeedingLayout and index counters. Clarified DimFill handling by continuing early if the DimFill is not valid or lacks a To property, reducing nesting and improving intent. Made minor formatting and code style improvements for consistency.

* Refactor subview filtering and sizing logic

Refactored repeated LINQ queries for subview filtering into reusable helper methods (`GetViewsThatMatch`, `GetViewsThatHavePos<TPos>`, `GetViewsThatHaveDim<TDim>`), reducing duplication and improving readability. Moved max content size calculations for various subview types into new helper methods (`GetMaxSizePos<TPos>`, `GetMaxSizeDim<TDim>`). Updated main logic to use these helpers. Adornment thickness calculation now uses a switch expression. These changes improve modularity and maintainability.

* Refactor subview categorization for layout calculation

Refactored layout calculation to use a single-pass CategorizeSubViews method, grouping subviews by relevant Pos/Dim types into a new CategorizedViews struct. This replaces multiple helper methods and reduces redundant iterations. Updated main logic to use these categorized lists, and unified size calculation helpers to further reduce code duplication. Improves performance and maintainability by consolidating subview processing and removing obsolete methods.

* Revert perf POC commits and add missing overrides to Combine types

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Add helper methods and simplify DimAuto.Calculate with foreach loops

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Refactor layout calculation in DimAuto.cs

Removed commented-out code and unnecessary list declarations to clean up the layout calculation logic.

* removed old plan file

* Code cleanup

* Add performance analysis and improvement plan for DimAuto.Calculate

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Add DimAuto benchmarks and benchmark documentation

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Implement Phase 1 & 2 performance optimizations for DimAuto.Calculate

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Code cleanup

* Delete plans/dimauto-perf-plan.md

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tig <585482+tig@users.noreply.github.com>
Co-authored-by: Tig <tig@users.noreply.github.com>

Author: Copilot

Terminal.Gui/ViewBase/Layout/Dim.cs

@@ -485,6 +485,44 @@ internal virtual int Calculate (int location, int superviewContentSize, View us,
     internal virtual int GetMinimumContribution (int location, int superviewContentSize, View us, Dimension dimension) =>
         Calculate (location, superviewContentSize, us, dimension);
 
+    /// <summary>
+    ///     Indicates whether this Dim has a fixed value that doesn't depend on layout calculations.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         This property is used by <see cref="DimAuto"/> to identify dimensions that can be
+    ///         determined without performing layout calculations on other views.
+    ///     </para>
+    ///     <para>
+    ///         Fixed dimensions include <see cref="DimAbsolute"/> and dimensions calculated by
+    ///         <see cref="DimFunc"/> that don't depend on other views' layouts.
+    ///     </para>
+    /// </remarks>
+    /// <returns>
+    ///     <see langword="true"/> if this Dim has a fixed value independent of layout;
+    ///     otherwise, <see langword="false"/>.
+    /// </returns>
+    internal virtual bool IsFixed => false;
+
+    /// <summary>
+    ///     Indicates whether this Dim requires the target view to be laid out before it can be calculated.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         This property is used by <see cref="DimAuto"/> to identify dimensions that depend on
+    ///         another view's layout being completed first.
+    ///     </para>
+    ///     <para>
+    ///         Dimensions that require target layout include <see cref="DimView"/> which depends on
+    ///         the target view's calculated size.
+    ///     </para>
+    /// </remarks>
+    /// <returns>
+    ///     <see langword="true"/> if this Dim requires the target view's layout to be calculated first;
+    ///     otherwise, <see langword="false"/>.
+    /// </returns>
+    internal virtual bool RequiresTargetLayout => false;
+
     #endregion virtual methods
 
     #region operators

Terminal.Gui/ViewBase/Layout/DimAbsolute.cs

@@ -23,4 +23,7 @@ public record DimAbsolute (int Size) : Dim
     internal override int GetAnchor (int size) => Size;
 
     internal override int Calculate (int location, int superviewContentSize, View us, Dimension dimension) => Math.Max (GetAnchor (0), 0);
+
+    /// <inheritdoc/>
+    internal override bool IsFixed => true;
 }