diff --git a/base/strings/annotated.jl b/base/strings/annotated.jl
index e7558a98479d2..65fd9cf485738 100644
--- a/base/strings/annotated.jl
+++ b/base/strings/annotated.jl
@@ -25,6 +25,17 @@ and a value (`Any`), paired together as a `Pair{Symbol, <:Any}`.
 Labels do not need to be unique, the same region can hold multiple annotations
 with the same label.
 
+Code written for `AnnotatedString`s in general should conserve the following
+properties:
+- Which characters an annotation is applied to
+- The order in which annotations are applied to each character
+
+Additional semantics may be introduced by specific uses of `AnnotatedString`s.
+
+A corollary of these rules is that adjacent, consecutively placed, annotations
+with identical labels and values are equivalent to a single annotation spanning
+the combined range.
+
 See also [`AnnotatedChar`](@ref), [`annotatedstring`](@ref),
 [`annotations`](@ref), and [`annotate!`](@ref).
 
@@ -273,36 +284,6 @@ annotatedstring(c::AnnotatedChar) =
 
 AnnotatedString(s::SubString{<:AnnotatedString}) = annotatedstring(s)
 
-"""
-    annotatedstring_optimize!(str::AnnotatedString)
-
-Merge contiguous identical annotations in `str`.
-"""
-function annotatedstring_optimize!(s::AnnotatedString)
-    last_seen = Dict{Pair{Symbol, Any}, Int}()
-    i = 1
-    while i <= length(s.annotations)
-        region, keyval = s.annotations[i]
-        prev = get(last_seen, keyval, 0)
-        if prev > 0
-            lregion, _ = s.annotations[prev]
-            if last(lregion) + 1 == first(region)
-                s.annotations[prev] =
-                    setindex(s.annotations[prev],
-                             first(lregion):last(region),
-                             1)
-                deleteat!(s.annotations, i)
-            else
-                delete!(last_seen, keyval)
-            end
-        else
-            last_seen[keyval] = i
-            i += 1
-        end
-    end
-    s
-end
-
 function repeat(str::AnnotatedString, r::Integer)
     r == 0 && return one(AnnotatedString)
     r == 1 && return str
@@ -310,19 +291,19 @@ function repeat(str::AnnotatedString, r::Integer)
     annotations = Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}()
     len = ncodeunits(str)
     fullregion = firstindex(str):lastindex(str)
-    for (region, annot) in str.annotations
-        if region == fullregion
-            push!(annotations, (firstindex(unannot):lastindex(unannot), annot))
+    if allequal(first, str.annotations) && first(first(str.annotations)) == fullregion
+        newfullregion = firstindex(unannot):lastindex(unannot)
+        for (_, annot) in str.annotations
+            push!(annotations, (newfullregion, annot))
         end
-    end
-    for offset in 0:len:(r-1)*len
-        for (region, annot) in str.annotations
-            if region != fullregion
+    else
+        for offset in 0:len:(r-1)*len
+            for (region, annot) in str.annotations
                 push!(annotations, (region .+ offset, annot))
             end
         end
     end
-    AnnotatedString(unannot, annotations) |> annotatedstring_optimize!
+    AnnotatedString(unannot, annotations)
 end
 
 repeat(str::SubString{<:AnnotatedString}, r::Integer) =
@@ -353,14 +334,9 @@ reverse(s::SubString{<:AnnotatedString}) = reverse(AnnotatedString(s))
 function _annotate!(annlist::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, range::UnitRange{Int}, @nospecialize(labelval::Pair{Symbol, <:Any}))
     label, val = labelval
     if val === nothing
-        indices = searchsorted(annlist, (range,), by=first)
-        labelindex = filter(i -> first(annlist[i][2]) === label, indices)
-        for index in Iterators.reverse(labelindex)
-            deleteat!(annlist, index)
-        end
+        deleteat!(annlist, findall(ann -> ann[1] == range && first(ann[2]) === label, annlist))
     else
-        sortedindex = searchsortedlast(annlist, (range,), by=first) + 1
-        insert!(annlist, sortedindex, (range, Pair{Symbol, Any}(label, val)))
+        push!(annlist, (range, Pair{Symbol, Any}(label, val)))
     end
 end
 
@@ -370,6 +346,9 @@ end
 
 Annotate a `range` of `str` (or the entire string) with a labeled value (`label` => `value`).
 To remove existing `label` annotations, use a value of `nothing`.
+
+The order in which annotations are applied to `str` is semantically meaningful,
+as described in [`AnnotatedString`](@ref).
 """
 annotate!(s::AnnotatedString, range::UnitRange{Int}, @nospecialize(labelval::Pair{Symbol, <:Any})) =
     (_annotate!(s.annotations, range, labelval); s)
@@ -402,6 +381,9 @@ annotations that overlap with `position` will be returned.
 Annotations are provided together with the regions they apply to, in the form of
 a vector of region–annotation tuples.
 
+In accordance with the semantics documented in [`AnnotatedString`](@ref), the
+order of annotations returned matches the order in which they were applied.
+
 See also: `annotate!`.
 """
 annotations(s::AnnotatedString) = s.annotations
@@ -536,10 +518,19 @@ function write(dest::AnnotatedIOBuffer, src::AnnotatedIOBuffer)
     nb
 end
 
+"""
+    _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, span::UnitRange{Int})
+
+Erase the presence of `annotations` within a certain `span`.
+
+This operates by removing all elements of `annotations` that are entirely
+contained in `span`, truncating ranges that partially overlap, and splitting
+annotations that subsume `span` to just exist either side of `span`.
+"""
 function _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, span::UnitRange{Int})
     # Clear out any overlapping pre-existing annotations.
     filter!(((region, _),) -> first(region) < first(span) || last(region) > last(span), annotations)
-    extras = Tuple{UnitRange{Int}, Pair{Symbol, Any}}[]
+    extras = Tuple{Int, Tuple{UnitRange{Int}, Pair{Symbol, Any}}}[]
     for i in eachindex(annotations)
         region, annot = annotations[i]
         # Test for partial overlap
@@ -550,31 +541,68 @@ function _clear_annotations_in_region!(annotations::Vector{Tuple{UnitRange{Int},
             # If `span` fits exactly within `region`, then we've only copied over
             # the beginning overhang, but also need to conserve the end overhang.
             if first(region) < first(span) && last(span) < last(region)
-                push!(extras, (last(span)+1:last(region), annot))
+                push!(extras, (i, (last(span)+1:last(region), annot)))
             end
         end
-        # Insert any extra entries in the appropriate position
-        for entry in extras
-            sortedindex = searchsortedlast(annotations, (first(entry),), by=first) + 1
-            insert!(annotations, sortedindex, entry)
-        end
+    end
+    # Insert any extra entries in the appropriate position
+    for (offset, (i, entry)) in enumerate(extras)
+        insert!(annotations, i + offset, entry)
     end
     annotations
 end
 
+"""
+    _insert_annotations!(io::AnnotatedIOBuffer, annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, offset::Int = position(io))
+
+Register new `annotations` in `io`, applying an `offset` to their regions.
+
+The largely consists of simply shifting the regions of `annotations` by `offset`
+and pushing them onto `io`'s annotations. However, when it is possible to merge
+the new annotations with recent annotations in accordance with the semantics
+outlined in [`AnnotatedString`](@ref), we do so. More specifically, when there
+is a run of the most recent annotations that are also present as the first
+`annotations`, with the same value and adjacent regions, the new annotations are
+merged into the existing recent annotations by simply extending their range.
+
+This is implemented so that one can say write an `AnnotatedString` to an
+`AnnotatedIOBuffer` one character at a time without needlessly producing a
+new annotation for each character.
+"""
 function _insert_annotations!(io::AnnotatedIOBuffer, annotations::Vector{Tuple{UnitRange{Int}, Pair{Symbol, Any}}}, offset::Int = position(io))
-    if !eof(io)
-        for (region, annot) in annotations
-            region = first(region)+offset:last(region)+offset
-            sortedindex = searchsortedlast(io.annotations, (region,), by=first) + 1
-            insert!(io.annotations, sortedindex, (region, annot))
-        end
-    else
-        for (region, annot) in annotations
-            region = first(region)+offset:last(region)+offset
-            push!(io.annotations, (region, annot))
+    run = 0
+    if !isempty(io.annotations) && last(first(last(io.annotations))) == offset
+        for i in reverse(axes(annotations, 1))
+            annot = annotations[i]
+            first(first(annot)) == 1 || continue
+            if last(annot) == last(last(io.annotations))
+                valid_run = true
+                for runlen in 1:i
+                    new_range, new_annot = annotations[begin+runlen-1]
+                    old_range, old_annot = io.annotations[end-i+runlen]
+                    if last(old_range) != offset || first(new_range) != 1 || old_annot != new_annot
+                        valid_run = false
+                        break
+                    end
+                end
+                if valid_run
+                    run = i
+                    break
+                end
+            end
         end
     end
+    for runindex in 0:run-1
+        old_index = lastindex(io.annotations) - run + 1 + runindex
+        old_region, annot = io.annotations[old_index]
+        new_region, _ = annotations[begin+runindex]
+        io.annotations[old_index] = (first(old_region):last(new_region)+offset, annot)
+    end
+    for index in run+1:lastindex(annotations)
+        region, annot = annotations[index]
+        start, stop = first(region), last(region)
+        push!(io.annotations, (start+offset:stop+offset, annot))
+    end
 end
 
 function read(io::AnnotatedIOBuffer, ::Type{AnnotatedString{T}}) where {T <: AbstractString}
diff --git a/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/md5 b/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/md5
new file mode 100644
index 0000000000000..758a74bce9dae
--- /dev/null
+++ b/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/md5
@@ -0,0 +1 @@
+6969fb6d2e8585d26beef865910ec8ef
diff --git a/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/sha512 b/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/sha512
new file mode 100644
index 0000000000000..3d1ac8791e14d
--- /dev/null
+++ b/deps/checksums/StyledStrings-ac472083359dde956aed8c61d43b8158ac84d9ce.tar.gz/sha512
@@ -0,0 +1 @@
+281292e8478d72ab66b84cbd4f42e5dc2dd5054e8c54a79de8f0c0537d28962b460e67fe71230ead6b02386b87d0423879d51ce53a2b2427ce55866d62d6ebde
diff --git a/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/md5 b/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/md5
deleted file mode 100644
index 511b60c7e2217..0000000000000
--- a/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/md5
+++ /dev/null
@@ -1 +0,0 @@
-8fc4fd7e90d35e7d8d06a6b7c312ec03
diff --git a/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/sha512 b/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/sha512
deleted file mode 100644
index d5e2302499a2c..0000000000000
--- a/deps/checksums/StyledStrings-bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b.tar.gz/sha512
+++ /dev/null
@@ -1 +0,0 @@
-137360872c9b75276426efa9e9096e442115a554b7e00dc98ce02904fa1a535f76e48ba1366fc517794490a494cccc3238d006ebb43dadb5594e5099a2c36f55
diff --git a/stdlib/StyledStrings.version b/stdlib/StyledStrings.version
index 8e489a5daf289..81a599f125406 100644
--- a/stdlib/StyledStrings.version
+++ b/stdlib/StyledStrings.version
@@ -1,4 +1,4 @@
 STYLEDSTRINGS_BRANCH = main
-STYLEDSTRINGS_SHA1 = bfdb4c3f73a93a956ad48b0f06f89eb1cd40ff6b
+STYLEDSTRINGS_SHA1 = ac472083359dde956aed8c61d43b8158ac84d9ce
 STYLEDSTRINGS_GIT_URL := https://github.com/JuliaLang/StyledStrings.jl.git
 STYLEDSTRINGS_TAR_URL = https://api.github.com/repos/JuliaLang/StyledStrings.jl/tarball/$1
diff --git a/test/strings/annotated.jl b/test/strings/annotated.jl
index 02325b5b8b2ee..13d463b4c7d21 100644
--- a/test/strings/annotated.jl
+++ b/test/strings/annotated.jl
@@ -28,10 +28,10 @@
     @test Base.AnnotatedString(str[3:4]) ==
         Base.AnnotatedString("me", [(1:2, :thing => 0x01), (1:2, :all => 0x03)])
     @test Base.AnnotatedString(str[3:6]) ==
-        Base.AnnotatedString("me s", [(1:2, :thing => 0x01), (1:4, :all => 0x03), (4:4, :other => 0x02)])
-    @test str == Base.AnnotatedString("some string", [(1:4, :thing => 0x01), (1:11, :all => 0x03), (6:11, :other => 0x02)])
+        Base.AnnotatedString("me s", [(1:2, :thing => 0x01), (4:4, :other => 0x02), (1:4, :all => 0x03)])
+    @test str == Base.AnnotatedString("some string", [(1:4, :thing => 0x01), (6:11, :other => 0x02), (1:11, :all => 0x03)])
     @test str != Base.AnnotatedString("some string")
-    @test str != Base.AnnotatedString("some string", [(1:1, :thing => 0x01), (6:6, :other => 0x02), (11:11, :all => 0x03)])
+    @test str != Base.AnnotatedString("some string", [(1:1, :thing => 0x01), (1:11, :all => 0x03), (6:6, :other => 0x02)])
     @test str != Base.AnnotatedString("some string", [(1:4, :thing => 0x11), (1:11, :all => 0x13), (6:11, :other => 0x12)])
     @test str != Base.AnnotatedString("some thingg", [(1:4, :thing => 0x01), (1:11, :all => 0x03), (6:11, :other => 0x02)])
     @test Base.AnnotatedString([Base.AnnotatedChar('a', [:a => 1]), Base.AnnotatedChar('b', [:b => 2])]) ==
@@ -55,15 +55,8 @@
     # @test collect(Base.eachstyle(str)) ==
     #     [("some", [:thing => 0x01, :all => 0x03]),
     #     (" string", [:all => 0x03, :other => 0x02])]
-    @test ==(Base.annotatedstring_optimize!(
-        Base.AnnotatedString("abc", [(1:1, :val => 1),
-                             (2:2, :val => 2),
-                             (2:2, :val => 1),
-                             (3:3, :val => 2)])),
-             Base.AnnotatedString("abc", [(1:2, :val => 1),
-                                  (2:3, :val => 2)]))
     @test chopprefix(sprint(show, str), "Base.") ==
-        "AnnotatedString{String}(\"some string\", [(1:4, :thing => 0x01), (1:11, :all => 0x03), (6:11, :other => 0x02)])"
+        "AnnotatedString{String}(\"some string\", [(1:4, :thing => 0x01), (6:11, :other => 0x02), (1:11, :all => 0x03)])"
     @test eval(Meta.parse(repr(str))) == str
     @test sprint(show, MIME("text/plain"), str) == "\"some string\""
 end
@@ -153,8 +146,8 @@ end
     # Check `annotate!`, including region sorting
     @test truncate(aio, 0).io.size == 0
     @test write(aio, "hello world") == ncodeunits("hello world")
-    @test Base.annotate!(aio, 7:11, :tag => 2) === aio
     @test Base.annotate!(aio, 1:5, :tag => 1) === aio
+    @test Base.annotate!(aio, 7:11, :tag => 2) === aio
     @test Base.annotations(aio) == [(1:5, :tag => 1), (7:11, :tag => 2)]
     # Reading
     @test read(seekstart(deepcopy(aio.io)), String) == "hello world"
@@ -182,24 +175,42 @@ end
     @test Base.annotations(aio) == [(1:5, :tag => 1), (7:11, :tag => 2)] # Should be unchanged
     @test write(seek(aio, 0), Base.AnnotatedString("hey-o", [(1:5, :hey => 'o')])) == 5
     @test read(seekstart(aio), String) == "hey-o alice"
-    @test Base.annotations(aio) == [(1:5, :hey => 'o'), (7:11, :tag => 2)] # First annotation should have been entirely replaced
+    @test Base.annotations(aio) == [(7:11, :tag => 2), (1:5, :hey => 'o')] # First annotation should have been entirely replaced
     @test write(seek(aio, 7), Base.AnnotatedString("bbi", [(1:3, :hey => 'a')])) == 3 # a[lic => bbi]e ('alice' => 'abbie')
     @test read(seekstart(aio), String) == "hey-o abbie"
-    @test Base.annotations(aio) == [(1:5, :hey => 'o'), (7:7, :tag => 2), (8:10, :hey => 'a'), (11:11, :tag => 2)]
+    @test Base.annotations(aio) == [(7:7, :tag => 2), (11:11, :tag => 2), (1:5, :hey => 'o'), (8:10, :hey => 'a')]
     @test write(seek(aio, 0), Base.AnnotatedString("ab")) == 2 # Check first annotation's region is adjusted correctly
     @test read(seekstart(aio), String) == "aby-o abbie"
-    @test Base.annotations(aio) == [(3:5, :hey => 'o'), (7:7, :tag => 2), (8:10, :hey => 'a'), (11:11, :tag => 2)]
+    @test Base.annotations(aio) == [(7:7, :tag => 2), (11:11, :tag => 2), (3:5, :hey => 'o'), (8:10, :hey => 'a')]
     @test write(seek(aio, 3), Base.AnnotatedString("ss")) == 2
     @test read(seekstart(aio), String) == "abyss abbie"
-    @test Base.annotations(aio) == [(3:3, :hey => 'o'), (7:7, :tag => 2), (8:10, :hey => 'a'), (11:11, :tag => 2)]
+    @test Base.annotations(aio) == [(7:7, :tag => 2), (11:11, :tag => 2), (3:3, :hey => 'o'), (8:10, :hey => 'a')]
     # Writing one buffer to another
     newaio = Base.AnnotatedIOBuffer()
     @test write(newaio, seekstart(aio)) == 11
     @test read(seekstart(newaio), String) == "abyss abbie"
     @test Base.annotations(newaio) == Base.annotations(aio)
     @test write(seek(newaio, 5), seek(aio, 5)) == 6
-    @test Base.annotations(newaio) == Base.annotations(aio)
+    @test sort(Base.annotations(newaio)) == sort(Base.annotations(aio))
     @test write(newaio, seek(aio, 5)) == 6
     @test read(seekstart(newaio), String) == "abyss abbie abbie"
-    @test Base.annotations(newaio) == vcat(Base.annotations(aio), [(13:13, :tag => 2), (14:16, :hey => 'a'), (17:17, :tag => 2)])
+    @test sort(Base.annotations(newaio)) == sort(vcat(Base.annotations(aio), [(13:13, :tag => 2), (14:16, :hey => 'a'), (17:17, :tag => 2)]))
+    # The `_insert_annotations!` cautious-merging optimisation
+    aio = Base.AnnotatedIOBuffer()
+    @test write(aio, Base.AnnotatedChar('a', [:a => 1, :b => 2])) == 1
+    @test Base.annotations(aio) == [(1:1, :a => 1), (1:1, :b => 2)]
+    @test write(aio, Base.AnnotatedChar('b', [:a => 1, :b => 2])) == 1
+    @test Base.annotations(aio) == [(1:2, :a => 1), (1:2, :b => 2)]
+    let aio2 = copy(aio) # A different start makes merging too risky to do.
+        @test write(aio2, Base.AnnotatedChar('c', [:a => 0, :b => 2])) == 1
+        @test Base.annotations(aio2) == [(1:2, :a => 1), (1:2, :b => 2), (3:3, :a => 0), (3:3, :b => 2)]
+    end
+    let aio2 = copy(aio) # Merging some run of the most recent annotations is fine though.
+        @test write(aio2, Base.AnnotatedChar('c', [:b => 2])) == 1
+        @test Base.annotations(aio2) == [(1:2, :a => 1), (1:3, :b => 2)]
+    end
+    let aio2 = copy(aio) # ...and any subsequent annotations after a matching run can just be copied over.
+        @test write(aio2, Base.AnnotatedChar('c', [:b => 2, :c => 3, :d => 4])) == 1
+        @test Base.annotations(aio2) == [(1:2, :a => 1), (1:3, :b => 2), (3:3, :c => 3), (3:3, :d => 4)]
+    end
 end