diff --git a/src/base/base_math.h b/src/base/base_math.h
index a8fe3002..57d03e2e 100644
--- a/src/base/base_math.h
+++ b/src/base/base_math.h
@@ -4,6 +4,19 @@
 #define Tau             ((f32)6.28318530717958647693)
 #define GoldenRatio     ((f32)1.61803398874989484820)
 
+////////////////////////////////
+//~ Axis types
+
+Enum(Axis)
+{
+    Axis_X = 0,
+    Axis_Y = 1,
+    Axis_Z = 2,
+
+    Axis_CountXY   = 2,
+    Axis_CountXYZ  = 3,
+};
+
 ////////////////////////////////
 //~ Vector types
 
diff --git a/src/ui/ui.c b/src/ui/ui.c
index 2e810bcb..403472d2 100644
--- a/src/ui/ui.c
+++ b/src/ui/ui.c
@@ -1,22 +1,5 @@
 UI_SharedState UI_shared_state = ZI;
 
-////////////////////////////////
-//~ State helpers
-
-UI_FiberState *UI_GetFiberState(void)
-{
-    UI_FiberState *f = UI_shared_state.fiber_states[FiberId()];
-    if (!f)
-    {
-        Arena *perm = PermArena();
-        PushAlign(perm, CachelineSize);
-        f = PushStruct(perm, UI_FiberState);
-        UI_shared_state.fiber_states[FiberId()] = f;
-        PushAlign(perm, CachelineSize);
-    }
-    return f;
-}
-
 ////////////////////////////////
 //~ Key helpers
 
@@ -25,8 +8,8 @@ UI_Key UI_KeyFromString(u64 seed, String str)
     /* TOIMPL */
     if (seed == 0)
     {
-        UI_FiberState *f = UI_GetFiberState();
-        seed = RandU64FromSeeds(f->top_parent->box->key.hash, f->top_tag->hash);
+        UI_SharedState *g = &UI_shared_state;
+        seed = RandU64FromSeeds(g->top_parent->box->key.hash, g->top_tag->hash);
     }
     UI_Key result = ZI;
     result.hash = HashFnv64(seed, str);
@@ -40,27 +23,27 @@ UI_Key UI_KeyFromString(u64 seed, String str)
 
 void UI_PushTagFromHash(u64 hash)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    if (f->top_tag)
+    UI_SharedState *g = &UI_shared_state;
+    if (g->top_tag)
     {
-        hash = RandU64FromSeeds(hash, f->top_tag->hash);
+        hash = RandU64FromSeeds(hash, g->top_tag->hash);
     }
-    UI_Tag *tag = PushStruct(f->build_arena, UI_Tag);
-    StackPush(f->top_tag, tag);
+    UI_Tag *tag = PushStruct(g->build_arena, UI_Tag);
+    StackPush(g->top_tag, tag);
     tag->hash = hash;
 }
 
 void UI_PushTagFromString(String str)
 {
-    UI_FiberState *f = UI_GetFiberState();
+    UI_SharedState *g = &UI_shared_state;
     u64 hash = Fnv64Basis;
-    if (f->top_tag)
+    if (g->top_tag)
     {
-        hash = f->top_tag->hash;
+        hash = g->top_tag->hash;
     }
     hash = HashFnv64(hash, str);
-    UI_Tag *tag = PushStruct(f->build_arena, UI_Tag);
-    StackPush(f->top_tag, tag);
+    UI_Tag *tag = PushStruct(g->build_arena, UI_Tag);
+    StackPush(g->top_tag, tag);
     tag->hash = hash;
 }
 
@@ -80,24 +63,24 @@ void UI_PushTagF_(char *fmt_cstr, ...)
 
 void UI_PopTag(void)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    StackPop(f->top_tag);
+    UI_SharedState *g = &UI_shared_state;
+    StackPop(g->top_tag);
 }
 
 //- Parent
 
 void UI_PushParent(UI_Box *box)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    UI_Parent *parent = PushStruct(f->build_arena, UI_Parent);
+    UI_SharedState *g = &UI_shared_state;
+    UI_Parent *parent = PushStruct(g->build_arena, UI_Parent);
     parent->box = box;
-    StackPush(f->top_parent, parent);
+    StackPush(g->top_parent, parent);
 }
 
 void UI_PopParent(void)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    StackPop(f->top_parent);
+    UI_SharedState *g = &UI_shared_state;
+    StackPop(g->top_parent);
 }
 
 ////////////////////////////////
@@ -105,13 +88,24 @@ void UI_PopParent(void)
 
 UI_Box *UI_BuildBox(UI_Flag flags, UI_Key key)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    UI_BoxBin *bin = &f->box_bins[key.hash % UI_NumBoxLookupBins];
-    UI_BoxBin *back_bin = &f->back_box_bins[key.hash % UI_NumBoxLookupBins];
+    UI_SharedState *g = &UI_shared_state;
+    UI_BoxBin *bin = &g->box_bins[key.hash % UI_NumBoxLookupBins];
+    UI_BoxBin *back_bin = &g->back_box_bins[key.hash % UI_NumBoxLookupBins];
 
-    UI_Box *box = PushStruct(f->build_arena, UI_Box);
+    UI_Box *box = PushStruct(g->build_arena, UI_Box);
     if (key.hash != 0)
     {
+#if RTC
+        /* Validate box not already built */
+        for (UI_Box *tmp = bin->first; tmp; tmp = tmp->next_in_bin)
+        {
+            if (tmp->key.hash == key.hash)
+            {
+                Assert(0);  /* Box with matching key already built */
+                break;
+            }
+        }
+#endif
         DllPushBackNP(bin->first, bin->last, box, next_in_bin, prev_in_bin);
     }
 
@@ -133,11 +127,13 @@ UI_Box *UI_BuildBox(UI_Flag flags, UI_Key key)
     {
         *box = *back_box;
     }
+    ++g->boxes_count;
 
     box->key = key;
     box->flags = flags;
-    box->parent = f->top_parent->box;
-    DllPushBack(f->top_parent->box->first, f->top_parent->box->last, box);
+    box->parent = g->top_parent->box;
+    DllPushBack(g->top_parent->box->first, g->top_parent->box->last, box);
+    ++box->parent->count;
 
     return box;
 }
@@ -147,8 +143,8 @@ UI_Box *UI_BuildBox(UI_Flag flags, UI_Key key)
 
 void UI_SetDisplayText(UI_Box *box, String str)
 {
-    UI_FiberState *f = UI_GetFiberState();
-    String text = PushString(f->build_arena, str);
+    UI_SharedState *g = &UI_shared_state;
+    String text = PushString(g->build_arena, str);
     box->display_text = text;
 }
 
@@ -165,38 +161,43 @@ void UI_SetDisplayImage(UI_Box *box, GPU_Resource *img)
 
 void UI_BeginBuild(void)
 {
-    UI_FiberState *f = UI_GetFiberState();
+    UI_SharedState *g = &UI_shared_state;
 
     /* Swap front & back build states */
     {
-        Arena *swp = f->build_arena;
-        f->build_arena = f->back_build_arena;
-        f->back_build_arena = swp;
+        Arena *swp = g->build_arena;
+        g->build_arena = g->back_build_arena;
+        g->back_build_arena = swp;
     }
-    f->back_box_bins = f->box_bins;
-    f->back_root_box = f->root_box;
+    g->back_box_bins = g->box_bins;
+    g->back_root_box = g->root_box;
+    g->back_boxes_count = g->boxes_count;
 
     /* Reset front build state */
-    if (!f->build_arena)
+    if (!g->build_arena)
     {
-        f->build_arena = AcquireArena(Gibi(64));
+        g->build_arena = AcquireArena(Gibi(64));
     }
-    ResetArena(f->build_arena);
+    ResetArena(g->build_arena);
+    g->boxes_count = 0;
 
     /* Init bins */
-    f->box_bins = PushStructs(f->build_arena, UI_BoxBin, UI_NumBoxLookupBins);
+    g->box_bins = PushStructs(g->build_arena, UI_BoxBin, UI_NumBoxLookupBins);
 
     /* Init root box */
-    f->root_box = PushStruct(f->build_arena, UI_Box);
-    f->root_box->key = UI_RootKey;
-    UI_BoxBin *bin = &f->box_bins[f->root_box->key.hash % UI_NumBoxLookupBins];
-    DllPushBackNP(bin->first, bin->last, f->root_box, next_in_bin, prev_in_bin);
+    g->root_box = PushStruct(g->build_arena, UI_Box);
+    g->root_box->key = UI_RootKey;
+    g->root_box->pref_size[Axis_X].kind = UI_SizeKind_Absolute;
+    g->root_box->pref_size[Axis_Y].kind = UI_SizeKind_Absolute;
+    UI_BoxBin *bin = &g->box_bins[g->root_box->key.hash % UI_NumBoxLookupBins];
+    DllPushBackNP(bin->first, bin->last, g->root_box, next_in_bin, prev_in_bin);
+    ++g->boxes_count;
 
     /* Init stacks */
     UI_PushTagF("root");
-    UI_PushParent(f->root_box);
+    UI_PushParent(g->root_box);
 
-    if (!f->back_build_arena)
+    if (!g->back_build_arena)
     {
         /* Back buffer not initialized, swap again */
         UI_BeginBuild();
@@ -208,39 +209,199 @@ void UI_BeginBuild(void)
 
 GPU_Resource *UI_EndBuild(void)
 {
-    UI_FiberState *f = UI_GetFiberState();
+    TempArena scratch = BeginScratchNoConflict();
+    UI_SharedState *g = &UI_shared_state;
 
     /* TODO: Ensure root is parent */
 
+    ////////////////////////////////
+    //- Layout
+
+    /* Init root size */
+    Vec2I32 root_size = VEC2I32(500, 500);
+    g->root_box->p0 = VEC2(0, 0);
+    g->root_box->p1 = VEC2(root_size.x, root_size.y);
+
+    /* Build pre-order & post-order box arrays */
+    u64 boxes_count = g->boxes_count;
+    UI_Box **boxes_pre = PushStructsNoZero(scratch.arena, UI_Box *, boxes_count);
+    UI_Box **boxes_post = PushStructsNoZero(scratch.arena, UI_Box *, boxes_count);
+    {
+        Struct(BoxNode) { BoxNode *next; b32 visited; UI_Box *box; };
+        BoxNode *first_dfs = 0;
+        u64 pre_index = 0;
+        u64 post_index = 0;
+        {
+            BoxNode *n = PushStruct(scratch.arena, BoxNode);
+            n->box = g->root_box;
+            StackPush(first_dfs, n);
+        }
+        while (first_dfs)
+        {
+            BoxNode *n = first_dfs;
+            UI_Box *box = n->box;
+            if (!n->visited)
+            {
+                /* Push children to dfs stack */
+                for (UI_Box *child = box->last; child; child = child->prev)
+                {
+                    BoxNode *child_n = PushStruct(scratch.arena, BoxNode);
+                    child_n->box = child;
+                    StackPush(first_dfs, child_n);
+                }
+                boxes_pre[pre_index++] = box;
+                n->visited = 1;
+            }
+            else
+            {
+                StackPop(first_dfs);
+                boxes_post[post_index++] = box;
+            }
+        }
+        Assert(pre_index == boxes_count);
+        Assert(post_index == boxes_count);
+    }
+
+    /* Calculate independent sizes */
+    for (u64 box_index = 1; box_index < boxes_count; ++box_index)
+    {
+        UI_Box *box = boxes_pre[box_index];
+        for (Axis axis = 0; axis < Axis_CountXY; ++axis)
+        {
+            UI_Size pref_size = box->pref_size[axis];
+            if (pref_size.kind == UI_SizeKind_Absolute)
+            {
+                box->solved_size[axis].v = pref_size.v;
+            }
+            else if (pref_size.kind == UI_SizeKind_Text)
+            {
+                /* FIXME: Calculate actual text size here */
+                f32 text_size = 10;
+                box->solved_size[axis].v = text_size;
+            }
+        }
+    }
+
+    /* Calculate upwards-dependent sizes */
+    for (u64 box_index = 0; box_index < boxes_count; ++box_index)
+    {
+        UI_Box *box = boxes_pre[box_index];
+        for (Axis axis = 0; axis < Axis_CountXY; ++axis)
+        {
+            UI_Size pref_size = box->pref_size[axis];
+            if (pref_size.kind == UI_SizeKind_Percent)
+            {
+                UI_Box *ancestor = box->parent;
+                UI_Size ancestor_size = ZI;
+                for (; ancestor; ancestor = ancestor->parent)
+                {
+                    UI_Size tmp = ancestor->solved_size[axis];
+                    if (tmp.kind == UI_SizeKind_Absolute || tmp.kind == UI_SizeKind_Text)
+                    {
+                        ancestor_size = tmp;
+                        break;
+                    }
+                }
+                // box->solved_size =
+            }
+        }
+    }
+
+    /* TODO: Remove this */
+#if 0
+    {
+        Struct(BoxNode) { BoxNode *next; UI_Box *box; };
+
+        BoxNode *first_dfs = 0;
+        BoxNode *first_dfs_reverse = 0;
+
+        /* Constrain layout downwards */
+        UI_Box *box = g->root_box;
+        while (box)
+        {
+            /* Push to reverse stack */
+            {
+                BoxNode *n = PushStruct(scratch.arena, BoxNode);
+                n->box = box;
+                StackPush(first_dfs_reverse, n);
+            }
+
+            {
+            }
+
+            /* Push children to dfs stack (in reverse) */
+            for (UI_Box *child = box->last; child; child = child->prev)
+            {
+                BoxNode *n = PushStruct(scratch.arena, BoxNode);
+                n->box = child;
+                StackPush(first_dfs, n);
+            }
+            /* Pop dfs stack */
+            if (first_dfs)
+            {
+                box = first_dfs->box;
+                StackPop(first_dfs);
+            }
+            else
+            {
+                box = 0;
+            }
+        }
+    }
+#endif
+
+    ////////////////////////////////
+    //- Build instances
+
     GPU_QueueKind render_queue = GPU_QueueKind_Direct;
     Fence *render_fence = GPU_FenceFromQueue(render_queue);
 
-    /* TODO: Real size */
-    Vec2I32 root_size = VEC2I32(500, 500);
+    /* Init render state */
+    if (!g->draw_rects_arena)
+    {
+        g->draw_rects_arena = AcquireArena(Gibi(64));
+    }
+
+    /* Build rect instance data */
+    for (u64 box_index = 0; box_index < boxes_count; ++box_index)
+    {
+        UI_Box *box = boxes_pre[box_index];
+        UI_RectInstance *rect = PushStruct(g->draw_rects_arena, UI_RectInstance);
+
+        rect->p0 = box->p0;
+        rect->p1 = box->p1;
+        rect->tint_srgb = 0xFFFFFFFF;
+    }
+
+    ////////////////////////////////
+    //- Render
 
     /* Acquire render target */
-    if (f->render_target && !EqVec2I32(root_size, GPU_GetTextureSize2D(f->render_target)))
+    if (g->render_target && !EqVec2I32(root_size, GPU_GetTextureSize2D(g->render_target)))
     {
-        YieldOnFence(render_fence, f->render_fence_target);
-        GPU_ReleaseResource(f->render_target, GPU_ReleaseFlag_None);
-        f->render_target = 0;
+        YieldOnFence(render_fence, g->render_fence_target);
+        GPU_ReleaseResource(g->render_target, GPU_ReleaseFlag_None);
+        g->render_target = 0;
     }
-    if (!f->render_target)
+    if (!g->render_target)
     {
         GPU_ResourceDesc desc = ZI;
         desc.kind = GPU_ResourceKind_Texture2D;
         desc.flags = GPU_ResourceFlag_Renderable;
         desc.texture.format = GPU_Format_R8G8B8A8_Unorm;
         desc.texture.size = VEC3I32(root_size.x, root_size.y, 1);
-        f->render_target = GPU_AcquireResource(desc);
+        g->render_target = GPU_AcquireResource(desc);
     }
 
     /* Build command list */
     GPU_CommandList *cl = GPU_BeginCommandList(render_queue);
     {
-
     }
-    f->render_fence_target = GPU_EndCommandList(cl);
+    g->render_fence_target = GPU_EndCommandList(cl);
 
-    return f->render_target;
+    /* Reset render state */
+    ResetArena(g->draw_rects_arena);
+
+    EndScratch(scratch);
+    return g->render_target;
 }
diff --git a/src/ui/ui.h b/src/ui/ui.h
index b39acfc8..7bef02d0 100644
--- a/src/ui/ui.h
+++ b/src/ui/ui.h
@@ -9,6 +9,23 @@ Struct(UI_Key)
     u64 hash;
 };
 
+////////////////////////////////
+//~ Size types
+
+Enum(UI_SizeKind)
+{
+    UI_SizeKind_Children,
+    UI_SizeKind_Percent,
+    UI_SizeKind_Absolute,
+    UI_SizeKind_Text,
+};
+
+Struct(UI_Size)
+{
+    UI_SizeKind kind;
+    f32 v;
+};
+
 ////////////////////////////////
 //~ Box types
 
@@ -21,20 +38,33 @@ Enum(UI_Flag)
 
 Struct(UI_Box)
 {
+    //- Hash list
     UI_Box *next_in_bin;
     UI_Box *prev_in_bin;
 
+    //- Tree
     UI_Box *parent;
     UI_Box *first;
     UI_Box *last;
     UI_Box *next;
     UI_Box *prev;
+    u64 count;
 
+    //- Persistent data
     UI_Key key;
-    UI_Flag flags;
 
+    //- Per-build data
+    UI_Flag flags;
     String display_text;
     GPU_Resource *display_image;
+    UI_Size pref_size[Axis_CountXY];
+
+    //- Solver data
+    UI_Size solved_size[Axis_CountXY];
+
+    //- Post-solve data
+    Vec2 p0;
+    Vec2 p1;
 };
 
 Struct(UI_BoxBin)
@@ -54,7 +84,7 @@ Struct(UI_Parent) { UI_Parent *next; UI_Box *box; };
 
 #define UI_NumBoxLookupBins 16384
 
-Struct(UI_FiberState)
+Struct(UI_SharedState)
 {
     //- Build state
     Arena *build_arena;
@@ -66,6 +96,9 @@ Struct(UI_FiberState)
     UI_Box *root_box;
     UI_Box *back_root_box;
 
+    u64 boxes_count;
+    u64 back_boxes_count;
+
     //- Stack state
     UI_Tag *top_tag;
     UI_Parent *top_parent;
@@ -73,18 +106,9 @@ Struct(UI_FiberState)
     //- Render state
     GPU_Resource *render_target;
     i64 render_fence_target;
-};
-
-Struct(UI_SharedState)
-{
-    UI_FiberState *fiber_states[MaxFibers];
+    Arena *draw_rects_arena;
 } extern UI_shared_state;
 
-////////////////////////////////
-//~ State helpers
-
-UI_FiberState *UI_GetFiberState(void);
-
 ////////////////////////////////
 //~ Key helpers
 
diff --git a/src/ui/ui_draw.h b/src/ui/ui_draw.h
index ab70bce6..bafb5ff1 100644
--- a/src/ui/ui_draw.h
+++ b/src/ui/ui_draw.h
@@ -16,7 +16,9 @@ AssertRootConst(UI_RectSig, 20);
 
 Struct(UI_RectInstance)
 {
-    i32 _;
+    Vec2 p0;
+    Vec2 p1;
+    u32 tint_srgb;
 };
 #define UI_DefaultRectInstance (UI_RectInstance) {  \
     0                                               \