diff --git a/src/AcDream.App/UI/MarkupDocument.cs b/src/AcDream.App/UI/MarkupDocument.cs
index 3d14d983..1ff22d9c 100644
--- a/src/AcDream.App/UI/MarkupDocument.cs
+++ b/src/AcDream.App/UI/MarkupDocument.cs
@@ -56,7 +56,31 @@ public static class MarkupDocument
             Height = F(root, "h"),
         };
 
+        // 2026-09-07 (docs/plans — owner direction "the size of the entire
+        // window needs to be enlarged for default and should also be
+        // resizeable"): a plugin panel is FIXED-SIZE by default —
+        // resizable="true" is the opt-in that arms real user drag-resize
+        // (both axes; UiRoot's generic edge/grip-drag mechanism already
+        // exists for every UiElement with Resizable=true — see
+        // UiElement.Resizable/ResizeX/ResizeY and RetailWindowManager.ResizeTo).
+        // minw/minh set the floor UiRoot's live drag and
+        // RetailWindowLayoutPersistence's restore clamp both already honor
+        // (UiElement.MinWidth/MinHeight); they default to the AUTHORED w/h so
+        // a resizable panel never shrinks below the layout its author tested.
+        bool resizable = B(root, "resizable", false);
+        panel.Resizable = resizable;
+        panel.MinWidth = FOr(root, "minw", panel.Width);
+        panel.MinHeight = FOr(root, "minh", panel.Height);
+        panel.ResizeX = resizable;
+        panel.ResizeY = resizable;
+
         // Optional per-window resize-axis lock: resize="x" | "y" | "both" | "none".
+        // Only meaningful once resizable="true" already armed the master
+        // switch above — Resizable=false (the default) blocks any drag-resize
+        // regardless of these axis flags, so this attribute alone can no
+        // longer make a panel resizable the way it silently could before
+        // resizable="true" existed (UiNineSlicePanel's own Resizable=true
+        // constructor default used to make the master switch a no-op).
         string? resize = (string?)root.Attribute("resize");
         if (resize is not null)
         {
@@ -141,7 +165,8 @@ public static class MarkupDocument
                         BarColor      = Color((string?)el.Attribute("color")),
                         Fill          = BindFloat((string?)el.Attribute("fill"), binding),
                         Label         = () => (cur(), max()) is (uint c, uint m) ? $"{c}/{m}" : null,
-                        Anchors       = Anchor((string?)el.Attribute("anchor")),
+                        // anchor= is applied uniformly for every element by
+                        // ApplyCommon below; no per-element handling needed here.
                         SpriteResolve = resolve,
                         BackLeft      = Hex((string?)el.Attribute("backleft")),
                         BackTile      = Hex((string?)el.Attribute("backtile")),
@@ -1137,6 +1162,18 @@ public static class MarkupDocument
     {
         element.Name = (string?)source.Attribute("name")
             ?? (string?)source.Attribute("id");
+
+        // 2026-09-07: anchor="left top right bottom" (space-separated; any
+        // subset; default "left top" — today's fixed placement) on ANY
+        // markup element. Semantics are identical to UiElement.Anchors/
+        // AnchorEdges/ApplyAnchor: "left right" stretches width with the
+        // parent, "top bottom" stretches height, "right" alone pins to the
+        // right edge at fixed width. A <group>'s own children resolve their
+        // anchor relative to the GROUP (their direct Parent), not the panel,
+        // because UiElement.ApplyAnchor always measures against Parent.Width/
+        // Height — no extra propagation code is needed for that.
+        element.Anchors = ParseAnchor((string?)source.Attribute("anchor"), source);
+
         BindBool((string?)source.Attribute("visible"), binding,
             value => element.Visible = value,
             sourceReader => element.VisibleSource = sourceReader);
@@ -1287,19 +1324,48 @@ public static class MarkupDocument
             System.Globalization.CultureInfo.InvariantCulture, out var v) ? v : 0u;
     }
 
-    private static AnchorEdges Anchor(string? csv)
+    /// <summary>
+    /// Parses <c>anchor="left top right bottom"</c> (space-separated, any
+    /// subset of the four tokens, case-insensitive) into <see cref="AnchorEdges"/>.
+    /// Absent/blank defaults to <c>Left | Top</c> — today's fixed top-left
+    /// placement, unchanged. An unrecognized token is a Build-time author
+    /// error, same "malformed markup throws" rule every other attribute in
+    /// this grammar follows (see e.g. <see cref="ValidateIconKind"/>) — the
+    /// message names the offending element via <see cref="ElementIdentity"/>
+    /// so a plugin author with several anchored siblings can find which one
+    /// is wrong.
+    /// </summary>
+    private static AnchorEdges ParseAnchor(string? tokens, XElement source)
     {
-        if (string.IsNullOrWhiteSpace(csv)) return AnchorEdges.Left | AnchorEdges.Top;
-        var a = AnchorEdges.None;
-        foreach (var part in csv.Split(',', System.StringSplitOptions.TrimEntries | System.StringSplitOptions.RemoveEmptyEntries))
-            a |= part.ToLowerInvariant() switch
+        if (string.IsNullOrWhiteSpace(tokens))
+            return AnchorEdges.Left | AnchorEdges.Top;
+
+        var edges = AnchorEdges.None;
+        foreach (string token in tokens.Split(
+            (char[]?)null, System.StringSplitOptions.RemoveEmptyEntries))
+        {
+            edges |= token.ToLowerInvariant() switch
             {
                 "left"   => AnchorEdges.Left,
                 "top"    => AnchorEdges.Top,
                 "right"  => AnchorEdges.Right,
                 "bottom" => AnchorEdges.Bottom,
-                _        => AnchorEdges.None,
+                _ => throw new FormatException(
+                    $"{ElementIdentity(source)} anchor=\"{tokens}\" has unknown token "
+                    + $"\"{token}\" (expected left, top, right, bottom)"),
             };
-        return a == AnchorEdges.None ? AnchorEdges.Left | AnchorEdges.Top : a;
+        }
+        return edges;
+    }
+
+    /// <summary>Identifies a markup element for a Build-time error message:
+    /// <c>&lt;button name="Foo"&gt;</c> when it carries a <c>name</c>/<c>id</c>,
+    /// else just <c>&lt;button&gt;</c>.</summary>
+    private static string ElementIdentity(XElement source)
+    {
+        string? name = (string?)source.Attribute("name") ?? (string?)source.Attribute("id");
+        return name is null
+            ? $"<{source.Name.LocalName}>"
+            : $"<{source.Name.LocalName} name=\"{name}\">";
     }
 }
diff --git a/tests/AcDream.App.Tests/UI/MarkupResizableAnchorTests.cs b/tests/AcDream.App.Tests/UI/MarkupResizableAnchorTests.cs
new file mode 100644
index 00000000..27feb01f
--- /dev/null
+++ b/tests/AcDream.App.Tests/UI/MarkupResizableAnchorTests.cs
@@ -0,0 +1,321 @@
+using System.Numerics;
+using AcDream.App.Rendering;
+using AcDream.App.Rendering.Gpu;
+using AcDream.App.Tests.Rendering.Gpu;
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+/// <summary>
+/// 2026-09-07 (owner direction: "The size of the entire window needs to be
+/// enlarged for default and should also be resizeable"): plugin markup's
+/// new <c>&lt;panel resizable="true" minw= minh=&gt;</c> grammar and the
+/// <c>anchor="left top right bottom"</c> attribute on <c>&lt;group&gt;</c>,
+/// <c>&lt;list&gt;</c>, <c>&lt;menu&gt;</c>, <c>&lt;field&gt;</c>,
+/// <c>&lt;label&gt;</c>, <c>&lt;button&gt;</c>, <c>&lt;icon&gt;</c>.
+/// Semantics are the existing <see cref="UiElement.Anchors"/>/
+/// <see cref="AnchorEdges"/>/<see cref="UiElement.ApplyAnchor"/> machinery —
+/// these tests prove MarkupDocument wires the two new attribute grammars
+/// into that machinery correctly, not the machinery itself (already covered
+/// by other UiElement anchor/resize tests).
+/// </summary>
+public sealed class MarkupResizableAnchorTests
+{
+    private sealed class ListBinding
+    {
+        public IReadOnlyList<string> Items => ["A", "B", "C"];
+        public int Selected { get; set; } = -1;
+        public Action<int> OnSelect => value => Selected = value;
+    }
+
+    // ── resizable / minw / minh parse tests ─────────────────────────────────
+
+    [Fact]
+    public void Build_PanelWithoutResizableAttribute_IsFixedSizeByDefault()
+    {
+        // The golden default: a panel that predates this feature (no
+        // resizable/minw/minh anywhere) must end up with the master switch
+        // OFF and both axes locked — this is the "exactly as today" contract
+        // item 2 of the plan requires.
+        const string xml = "<panel x=\"0\" y=\"0\" w=\"300\" h=\"200\"></panel>";
+        var panel = MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32));
+
+        Assert.False(panel.Resizable);
+        Assert.False(panel.ResizeX);
+        Assert.False(panel.ResizeY);
+        Assert.Equal(300f, panel.MinWidth);
+        Assert.Equal(200f, panel.MinHeight);
+    }
+
+    [Fact]
+    public void Build_PanelResizableTrue_ArmsBothAxesAndDefaultsMinToAuthoredSize()
+    {
+        const string xml = "<panel x=\"0\" y=\"0\" w=\"300\" h=\"200\" resizable=\"true\"></panel>";
+        var panel = MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32));
+
+        Assert.True(panel.Resizable);
+        Assert.True(panel.ResizeX);
+        Assert.True(panel.ResizeY);
+        Assert.Equal(300f, panel.MinWidth);
+        Assert.Equal(200f, panel.MinHeight);
+    }
+
+    [Fact]
+    public void Build_PanelResizableTrueWithMinwMinh_OverridesTheAuthoredSizeFloor()
+    {
+        const string xml =
+            "<panel x=\"0\" y=\"0\" w=\"300\" h=\"200\" resizable=\"true\" minw=\"150\" minh=\"90\"></panel>";
+        var panel = MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32));
+
+        Assert.Equal(150f, panel.MinWidth);
+        Assert.Equal(90f, panel.MinHeight);
+    }
+
+    [Fact]
+    public void Build_PanelResizableTrueWithResizeAxisLock_NarrowsToOneAxis()
+    {
+        // The pre-existing resize="x"|"y"|"both"|"none" attribute still
+        // layers on top of resizable="true" to narrow which axis actually
+        // drags — it just can no longer be the SOLE switch (resizable is).
+        const string xml =
+            "<panel x=\"0\" y=\"0\" w=\"300\" h=\"200\" resizable=\"true\" resize=\"x\"></panel>";
+        var panel = MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32));
+
+        Assert.True(panel.Resizable);
+        Assert.True(panel.ResizeX);
+        Assert.False(panel.ResizeY);
+    }
+
+    // ── anchor grammar parse tests ───────────────────────────────────────────
+
+    [Theory]
+    [InlineData("group")]
+    [InlineData("list")]
+    [InlineData("menu")]
+    [InlineData("field")]
+    [InlineData("label")]
+    [InlineData("button")]
+    [InlineData("icon")]
+    public void Build_ElementWithoutAnchorAttribute_DefaultsToLeftTop(string tag)
+    {
+        string xml = WrapSingle(tag, anchor: null);
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+
+        UiElement element = panel.Children[0];
+        Assert.Equal(AnchorEdges.Left | AnchorEdges.Top, element.Anchors);
+    }
+
+    [Theory]
+    [InlineData("group")]
+    [InlineData("list")]
+    [InlineData("menu")]
+    [InlineData("field")]
+    [InlineData("label")]
+    [InlineData("button")]
+    [InlineData("icon")]
+    public void Build_ElementAnchorLeftRight_SetsBothHorizontalEdges(string tag)
+    {
+        string xml = WrapSingle(tag, anchor: "left right");
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+
+        UiElement element = panel.Children[0];
+        Assert.Equal(AnchorEdges.Left | AnchorEdges.Right, element.Anchors);
+    }
+
+    [Fact]
+    public void Build_AnchorAllFourTokens_SetsEveryEdge()
+    {
+        string xml = WrapSingle("group", anchor: "left top right bottom");
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+
+        Assert.Equal(
+            AnchorEdges.Left | AnchorEdges.Top | AnchorEdges.Right | AnchorEdges.Bottom,
+            panel.Children[0].Anchors);
+    }
+
+    [Fact]
+    public void Build_AnchorIsCaseInsensitiveAndOrderIndependent()
+    {
+        string xml = WrapSingle("button", anchor: "BOTTOM Right");
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+
+        Assert.Equal(AnchorEdges.Bottom | AnchorEdges.Right, panel.Children[0].Anchors);
+    }
+
+    [Fact]
+    public void Build_UnknownAnchorToken_ThrowsNamingTheElement()
+    {
+        const string xml =
+            "<panel x=\"0\" y=\"0\" w=\"100\" h=\"60\">" +
+            "<button name=\"Fire1\" x=\"0\" y=\"0\" w=\"40\" h=\"20\" text=\"Go\" anchor=\"left frotz\"/>" +
+            "</panel>";
+
+        FormatException ex = Assert.Throws<FormatException>(
+            () => MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32)));
+
+        Assert.Contains("Fire1", ex.Message);
+        Assert.Contains("frotz", ex.Message);
+    }
+
+    private static string WrapSingle(string tag, string? anchor)
+    {
+        string anchorAttr = anchor is null ? string.Empty : $" anchor=\"{anchor}\"";
+        string inner = tag switch
+        {
+            "group" => $"<group x=\"10\" y=\"10\" w=\"100\" h=\"60\"{anchorAttr}></group>",
+            "list" => $"<list x=\"10\" y=\"10\" w=\"100\" h=\"60\" items=\"{{Items}}\" " +
+                      $"selected=\"{{Selected}}\" onchange=\"{{OnSelect}}\"{anchorAttr}/>",
+            "menu" => $"<menu x=\"10\" y=\"10\" w=\"100\" h=\"20\" items=\"{{Items}}\"{anchorAttr}/>",
+            "field" => $"<field x=\"10\" y=\"10\" w=\"100\" h=\"20\"{anchorAttr}/>",
+            "label" => $"<label x=\"10\" y=\"10\" text=\"Hi\"{anchorAttr}/>",
+            "button" => $"<button x=\"10\" y=\"10\" w=\"40\" h=\"20\" text=\"Go\"{anchorAttr}/>",
+            "icon" => $"<icon x=\"10\" y=\"10\" w=\"32\" h=\"32\" did=\"0x06000001\"{anchorAttr}/>",
+            _ => throw new ArgumentOutOfRangeException(nameof(tag)),
+        };
+        return "<panel x=\"0\" y=\"0\" w=\"200\" h=\"150\">" + inner + "</panel>";
+    }
+
+    // ── dynamic anchor re-layout tests (recording renderer) ─────────────────
+
+    private sealed class NullGpuFrameSource : ICurrentGpuFrameSource
+    {
+        public IGpuFrame? CurrentFrame => null;
+    }
+
+    private static UiRenderContext MakeContext(float w, float h)
+    {
+        var device = new RecordingGpuDevice();
+        var renderer = new TextRenderer(device, new NullGpuFrameSource(), "unused");
+        renderer.Begin(new Vector2(w, h));
+        return new UiRenderContext(renderer, new Vector2(w, h));
+    }
+
+    [Fact]
+    public void ResizingPanel_LeftRightList_WidensWithThePanel()
+    {
+        const string xml = """
+            <panel x="0" y="0" w="300" h="200" resizable="true" minw="200" minh="150">
+              <list anchor="left right" x="10" y="10" w="280" h="150"
+                    items="{Items}" selected="{Selected}" onchange="{OnSelect}"/>
+            </panel>
+            """;
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+        var list = Assert.IsType<UiMarkupList>(panel.Children[0]);
+
+        UiRenderContext ctx = MakeContext(600f, 400f);
+        // First draw at the authored width captures the list's 10px margin to
+        // each side (10 = 300 - (10 + 280)) as its anchor baseline.
+        panel.DrawSelfAndChildren(ctx);
+        Assert.Equal(280f, list.Width);
+
+        // A live drag-resize (RetailWindowManager.ResizeTo) mutates Width
+        // directly; the next draw re-applies the captured 10px margins
+        // against the NEW panel width.
+        panel.Width = 500f;
+        panel.DrawSelfAndChildren(ctx);
+
+        Assert.Equal(480f, list.Width); // 500 - 10 - 10
+        Assert.Equal(10f, list.Left);   // left margin preserved
+    }
+
+    [Fact]
+    public void ResizingPanel_RightAnchoredButton_MovesWithTheRightEdge()
+    {
+        const string xml = """
+            <panel x="0" y="0" w="300" h="200" resizable="true" minw="200" minh="150">
+              <button anchor="right" x="250" y="10" w="40" h="20" text="X"/>
+            </panel>
+            """;
+        var panel = MarkupDocument.Build(xml, new object(), _ => (1u, 32, 32));
+        var button = Assert.IsType<UiSimpleButton>(panel.Children[0]);
+
+        UiRenderContext ctx = MakeContext(600f, 400f);
+        panel.DrawSelfAndChildren(ctx);
+        Assert.Equal(250f, button.Left); // unchanged: 10px margin to the right edge
+        Assert.Equal(40f, button.Width); // fixed width — right-only anchor never stretches
+
+        panel.Width = 500f;
+        panel.DrawSelfAndChildren(ctx);
+
+        Assert.Equal(450f, button.Left); // 500 - 10 - 40: follows the right edge
+        Assert.Equal(40f, button.Width); // still fixed width
+    }
+
+    [Fact]
+    public void ResizingPanel_GroupStretchesBothAxesAndNestedListFollowsGroupWidth()
+    {
+        // Group anchors to every edge (generalizing "top bottom stretches" to
+        // both axes so this one test can drive BOTH panel dimensions), and
+        // its nested list anchors left+right RELATIVE TO THE GROUP — proving
+        // "groups propagate to children" (item 1): the list's own margins are
+        // captured against the group's Width, not the panel's.
+        const string xml = """
+            <panel x="0" y="0" w="300" h="200" resizable="true" minw="200" minh="150">
+              <group anchor="left top right bottom" x="10" y="10" w="280" h="180">
+                <list anchor="left right" x="5" y="5" w="270" h="170"
+                      items="{Items}" selected="{Selected}" onchange="{OnSelect}"/>
+              </group>
+            </panel>
+            """;
+        var panel = MarkupDocument.Build(xml, new ListBinding(), _ => (1u, 32, 32));
+        var group = Assert.IsType<UiPanel>(panel.Children[0]);
+        var list = Assert.IsType<UiMarkupList>(group.Children[0]);
+
+        UiRenderContext ctx = MakeContext(600f, 400f);
+        panel.DrawSelfAndChildren(ctx);
+        Assert.Equal(280f, group.Width);
+        Assert.Equal(180f, group.Height);
+        Assert.Equal(270f, list.Width);
+
+        panel.Width = 500f;
+        panel.Height = 300f;
+        panel.DrawSelfAndChildren(ctx);
+
+        // Group stretches on both axes (10px margin preserved on every side).
+        Assert.Equal(480f, group.Width);  // 500 - 10 - 10
+        Assert.Equal(280f, group.Height); // 300 - 10 - 10
+        // The nested list follows the GROUP's new width (5px margin to each
+        // side of the group, not the panel).
+        Assert.Equal(470f, list.Width);   // 480 - 5 - 5
+    }
+
+    // ── golden: a plain panel with none of the new attributes is unaffected ──
+
+    [Fact]
+    public void Build_PlainPanel_DrawsIdenticallyAcrossRepeatedBuilds()
+    {
+        // No resizable/minw/minh/anchor anywhere — the pre-existing markup
+        // shape every current plugin panel uses today. Two independent
+        // builds + draws of the identical markup must produce byte-identical
+        // recorded GPU call sequences and geometry, proving the new
+        // ApplyCommon anchor-parsing code path is a no-op on the default
+        // path this feature must not disturb.
+        const string xml = """
+            <panel x="0" y="0" w="200" h="120" title="V">
+              <group x="4" y="4" w="180" h="40" background="#FF102030">
+                <button x="4" y="4" w="60" h="20" text="Go"/>
+                <label x="4" y="28" text="Hi"/>
+              </group>
+              <list x="4" y="48" w="180" h="60" items="{Items}" selected="{Selected}" onchange="{OnSelect}"/>
+            </panel>
+            """;
+
+        var panelA = MarkupDocument.Build(xml, new ListBinding(), _ => (7u, 32, 32));
+        var panelB = MarkupDocument.Build(xml, new ListBinding(), _ => (7u, 32, 32));
+
+        var deviceA = new RecordingGpuDevice();
+        var rendererA = new TextRenderer(deviceA, new NullGpuFrameSource(), "unused");
+        rendererA.Begin(new Vector2(400f, 300f));
+        panelA.DrawSelfAndChildren(new UiRenderContext(rendererA, new Vector2(400f, 300f)));
+
+        var deviceB = new RecordingGpuDevice();
+        var rendererB = new TextRenderer(deviceB, new NullGpuFrameSource(), "unused");
+        rendererB.Begin(new Vector2(400f, 300f));
+        panelB.DrawSelfAndChildren(new UiRenderContext(rendererB, new Vector2(400f, 300f)));
+
+        Assert.Equal(deviceA.Calls, deviceB.Calls);
+        Assert.False(panelA.Resizable);
+        Assert.Equal(AnchorEdges.Left | AnchorEdges.Top, panelA.Children[0].Anchors);
+        Assert.Equal(AnchorEdges.Left | AnchorEdges.Top, panelB.Children[0].Anchors);
+    }
+}