diff --git a/src/FastAlmostBandedMatrices.jl b/src/FastAlmostBandedMatrices.jl
index 29c56e8..b95c4c6 100644
--- a/src/FastAlmostBandedMatrices.jl
+++ b/src/FastAlmostBandedMatrices.jl
@@ -14,6 +14,79 @@ import MatrixFactorizations: QR, QRPackedQ, getQ, getR, QRPackedQLayout, AdjQRPa
 
 @reexport using BandedMatrices
 
+# ------------------
+# DisjointRange - for zero-allocation colsupport
+# ------------------
+
+"""
+    DisjointRange{T}
+
+A lazy representation of the union of two ranges, supporting iteration and indexing
+without heap allocation.
+"""
+struct DisjointRange{T<:Integer, R1<:AbstractUnitRange{T}, R2<:AbstractUnitRange{T}} <:
+       AbstractVector{T}
+    r1::R1
+    r2::R2
+end
+
+Base.size(d::DisjointRange) = (length(d.r1) + length(d.r2),)
+Base.length(d::DisjointRange) = length(d.r1) + length(d.r2)
+
+@inline function Base.getindex(d::DisjointRange, i::Integer)
+    @boundscheck checkbounds(d, i)
+    n1 = length(d.r1)
+    if i <= n1
+        return @inbounds d.r1[i]
+    else
+        return @inbounds d.r2[i - n1]
+    end
+end
+
+Base.IndexStyle(::Type{<:DisjointRange}) = IndexLinear()
+
+@inline function Base.iterate(d::DisjointRange)
+    if !isempty(d.r1)
+        val, state = iterate(d.r1)
+        return val, (1, state)
+    elseif !isempty(d.r2)
+        val, state = iterate(d.r2)
+        return val, (2, state)
+    else
+        return nothing
+    end
+end
+
+@inline function Base.iterate(d::DisjointRange, state)
+    which, inner_state = state
+    if which == 1
+        next = iterate(d.r1, inner_state)
+        if next !== nothing
+            return next[1], (1, next[2])
+        else
+            # Switch to r2
+            if !isempty(d.r2)
+                val, new_state = iterate(d.r2)
+                return val, (2, new_state)
+            else
+                return nothing
+            end
+        end
+    else
+        next = iterate(d.r2, inner_state)
+        if next !== nothing
+            return next[1], (2, next[2])
+        else
+            return nothing
+        end
+    end
+end
+
+Base.first(d::DisjointRange) = isempty(d.r1) ? first(d.r2) : first(d.r1)
+Base.last(d::DisjointRange) = isempty(d.r2) ? last(d.r1) : last(d.r2)
+Base.minimum(d::DisjointRange) = min(minimum(d.r1), minimum(d.r2))
+Base.maximum(d::DisjointRange) = max(maximum(d.r1), maximum(d.r2))
+
 # ------------------
 # AlmostBandedMatrix
 # ------------------
@@ -138,7 +211,7 @@ end
 function Base.fill!(A::AlmostBandedMatrix, v)
     fill!(bandpart(A), v)
     fill!(fillpart(A), v)
-    return nothing
+    return A
 end
 
 @inline function colsupport(::AbstractAlmostBandedLayout, A, j)
@@ -151,7 +224,8 @@ end
         if isempty(sup)
             return Base.OneTo(r)
         else
-            return vcat(Base.OneTo(min(r, minimum(sup) - 1)), sup)
+            # Use DisjointRange to avoid heap allocation from vcat
+            return DisjointRange(Base.OneTo(min(r, minimum(sup) - 1)), sup)
         end
     end
 end
diff --git a/test/Project.toml b/test/Project.toml
index bb5b1cf..0c703ae 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -1,6 +1,8 @@
 [deps]
+AllocCheck = "9b6a8646-10ed-4001-bbdc-1d2f46dfbb1a"
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"
+BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 MatrixFactorizations = "a3b82374-2e81-5b9e-98ce-41277c0e4c87"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
diff --git a/test/alloc_tests.jl b/test/alloc_tests.jl
new file mode 100644
index 0000000..d826b2c
--- /dev/null
+++ b/test/alloc_tests.jl
@@ -0,0 +1,134 @@
+using AllocCheck
+using BenchmarkTools
+using FastAlmostBandedMatrices
+using FastAlmostBandedMatrices: DisjointRange
+using ArrayLayouts: colsupport, rowsupport
+using Test
+
+@testset "Allocation Tests" begin
+    @testset "DisjointRange - Zero Allocations" begin
+        # Test that DisjointRange operations don't allocate
+        r1 = Base.OneTo(5)
+        r2 = 10:15
+        dr = DisjointRange(r1, r2)
+
+        # Test length
+        allocs = @allocated length(dr)
+        @test allocs == 0
+
+        # Test getindex
+        allocs = @allocated dr[3]
+        @test allocs == 0
+
+        allocs = @allocated dr[8]
+        @test allocs == 0
+
+        # Test first/last
+        allocs = @allocated first(dr)
+        @test allocs == 0
+
+        allocs = @allocated last(dr)
+        @test allocs == 0
+
+        # Test iteration (after warmup)
+        sum_test = 0
+        for x in dr
+            sum_test += x
+        end
+        allocs = @allocated begin
+            s = 0
+            for x in dr
+                s += x
+            end
+            s
+        end
+        @test allocs == 0
+    end
+
+    @testset "colsupport - Zero Allocations" begin
+        n = 100
+        m = 2
+        B = brand(Float64, n, n, m + 1, m)
+        F = rand(Float64, m, n)
+        A = AlmostBandedMatrix(B, F)
+
+        # Warmup
+        colsupport(A, 5)
+        colsupport(A, 50)
+
+        # Test colsupport for j <= l+u (should return OneTo, no allocation)
+        allocs = @allocated colsupport(A, 5)
+        @test allocs == 0
+
+        # Test colsupport for j > l+u (now returns DisjointRange instead of vcat)
+        allocs = @allocated colsupport(A, 50)
+        @test allocs == 0
+    end
+
+    @testset "rowsupport - Zero Allocations" begin
+        n = 100
+        m = 2
+        B = brand(Float64, n, n, m + 1, m)
+        F = rand(Float64, m, n)
+        A = AlmostBandedMatrix(B, F)
+
+        # Warmup
+        rowsupport(A, 1)
+        rowsupport(A, 50)
+
+        # Test rowsupport (always returns UnitRange, no allocation)
+        allocs = @allocated rowsupport(A, 1)
+        @test allocs == 0
+
+        allocs = @allocated rowsupport(A, 50)
+        @test allocs == 0
+    end
+
+    @testset "getindex/setindex! - Zero Allocations" begin
+        n = 100
+        m = 2
+        B = brand(Float64, n, n, m + 1, m)
+        F = rand(Float64, m, n)
+        A = AlmostBandedMatrix(B, F)
+
+        # Warmup
+        _ = A[50, 50]
+        A[50, 50] = 1.0
+
+        # Test getindex
+        allocs = @allocated A[50, 50]
+        @test allocs == 0
+
+        # Test setindex! in band part
+        allocs = @allocated A[50, 50] = 2.0
+        @test allocs == 0
+
+        # Test setindex! in fill part
+        allocs = @allocated A[1, 50] = 3.0
+        @test allocs == 0
+
+        # Test setindex! in overlapping part
+        allocs = @allocated A[1, 1] = 4.0
+        @test allocs == 0
+    end
+
+    @testset "bandpart/fillpart - Zero Allocations" begin
+        n = 100
+        m = 2
+        B = brand(Float64, n, n, m + 1, m)
+        F = rand(Float64, m, n)
+        A = AlmostBandedMatrix(B, F)
+
+        # Warmup
+        bandpart(A)
+        fillpart(A)
+
+        # Test bandpart
+        allocs = @allocated bandpart(A)
+        @test allocs == 0
+
+        # Test fillpart
+        allocs = @allocated fillpart(A)
+        @test allocs == 0
+    end
+end
diff --git a/test/runtests.jl b/test/runtests.jl
index f2941be..233ef38 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -107,3 +107,8 @@ using SafeTestsets, Test
         @test length(A1.fill.nzval) == 2
     end
 end
+
+# Allocation tests run separately to avoid precompilation interference
+if get(ENV, "GROUP", "all") == "all" || get(ENV, "GROUP", "all") == "nopre"
+    include("alloc_tests.jl")
+end