diff --git a/baremetal/shell/read.mu b/baremetal/shell/read.mu
index 5ebf98db..dcb5b070 100644
--- a/baremetal/shell/read.mu
+++ b/baremetal/shell/read.mu
@@ -1,8 +1,7 @@
 # out is not allocated
 fn read-cell in: (addr gap-buffer), out: (addr handle cell), trace: (addr trace) {
-  trace-text trace, "read", ""
-  trace-lower trace
   trace-text trace, "read", "tokenize"
+  trace-lower trace
   rewind-gap-buffer in
   var token-storage: (stream byte 0x1000)  # strings can be large
   var token/ecx: (addr stream byte) <- address token-storage
@@ -17,13 +16,12 @@ fn read-cell in: (addr gap-buffer), out: (addr handle cell), trace: (addr trace)
     read-symbol token, out
     loop
   }
-  trace-higher trace  # tokenize
   # TODO:
   #   insert parens
   #   transform infix
   #   token tree
   #   syntax tree
-  trace-higher trace  # read
+  trace-higher trace
 }
 
 fn next-token in: (addr gap-buffer), out: (addr stream byte), trace: (addr trace) {
diff --git a/baremetal/shell/sandbox.mu b/baremetal/shell/sandbox.mu
index 92e88fc8..6d5f64db 100644
--- a/baremetal/shell/sandbox.mu
+++ b/baremetal/shell/sandbox.mu
@@ -16,7 +16,7 @@ fn initialize-sandbox _self: (addr sandbox) {
   var trace-ah/eax: (addr handle trace) <- get self, trace
   allocate trace-ah
   var trace/eax: (addr trace) <- lookup *trace-ah
-  initialize-trace trace, 0x100/lines
+  initialize-trace trace, 0x100/lines, 0x10/visible-lines
 }
 
 ## some helpers for tests
@@ -176,6 +176,7 @@ fn run in: (addr gap-buffer), out: (addr stream byte), trace: (addr trace) {
   }
   # TODO: eval
   print-cell read-result, out
+  mark-lines-dirty trace
 }
 
 fn test-run-integer {
diff --git a/baremetal/shell/trace.mu b/baremetal/shell/trace.mu
index 5e0a2003..231fa2c4 100644
--- a/baremetal/shell/trace.mu
+++ b/baremetal/shell/trace.mu
@@ -2,22 +2,38 @@
 # An integral part of the Mu Shell is facilities for browsing traces.
 
 type trace {
+  # steady-state life cycle of a trace:
+  #   reload loop:
+  #     there are already some visible lines
+  #     append a bunch of new trace lines to the trace
+  #     render loop:
+  #       rendering displays trace lines that match visible lines
+  #       rendering computes cursor-line based on the cursor-y coordinate
+  #       edit-trace updates cursor-y coordinate
+  #       edit-trace might add/remove lines to visible
   curr-depth: int  # depth that will be assigned to next line appended
   data: (handle array trace-line)
   first-free: int
+  visible: (handle array trace-line)
+  recompute-visible?: boolean
+  top-line-index: int  # index into data
   cursor-y: int  # row index on screen
+  cursor-line-index: int  # index into data
 }
 
 type trace-line {
   depth: int
   label: (handle array byte)
   data: (handle array byte)
+  visible?: boolean
 }
 
-fn initialize-trace _self: (addr trace), capacity: int {
-  var self/eax: (addr trace) <- copy _self
+fn initialize-trace _self: (addr trace), capacity: int, visible-capacity: int {
+  var self/esi: (addr trace) <- copy _self
   var trace-ah/eax: (addr handle array trace-line) <- get self, data
   populate trace-ah, capacity
+  var visible-ah/eax: (addr handle array trace-line) <- get self, visible
+  populate visible-ah, visible-capacity
 }
 
 fn clear-trace _self: (addr trace) {
@@ -27,6 +43,18 @@ fn clear-trace _self: (addr trace) {
   # might leak memory; existing elements won't be used anymore
 }
 
+fn mark-lines-dirty _self: (addr trace) {
+  var self/eax: (addr trace) <- copy _self
+  var dest/edx: (addr boolean) <- get self, recompute-visible?
+  copy-to *dest, 1/true
+}
+
+fn mark-lines-clean _self: (addr trace) {
+  var self/eax: (addr trace) <- copy _self
+  var dest/edx: (addr boolean) <- get self, recompute-visible?
+  copy-to *dest, 0/false
+}
+
 fn has-errors? _self: (addr trace) -> _/eax: boolean {
   var self/eax: (addr trace) <- copy _self
   var max/edx: (addr int) <- get self, first-free
@@ -118,35 +146,40 @@ fn render-trace screen: (addr screen), _self: (addr trace), xmin: int, ymin: int
     compare i, max
     break-if->=
     $render-trace:iter: {
-      var offset/edx: (offset trace-line) <- compute-offset trace, i
-      var curr/edx: (addr trace-line) <- index trace, offset
+      var offset/ebx: (offset trace-line) <- compute-offset trace, i
+      var curr/ebx: (addr trace-line) <- index trace, offset
       var curr-label-ah/eax: (addr handle array byte) <- get curr, label
       var curr-label/eax: (addr array byte) <- lookup *curr-label-ah
       var bg/edi: int <- copy 0/black
       compare show-cursor?, 0/false
       {
         break-if-=
-        var self/eax: (addr trace) <- copy _self
         var cursor-y/eax: (addr int) <- get self, cursor-y
         compare *cursor-y, y
         break-if-!=
         bg <- copy 7/grey
+        var cursor-line-index/eax: (addr int) <- get self, cursor-line-index
+        copy-to *cursor-line-index, i
       }
       # always display errors
       var is-error?/eax: boolean <- string-equal? curr-label, "error"
       {
         compare is-error?, 0/false
         break-if-=
-        var curr-data-ah/eax: (addr handle array byte) <- get curr, data
-        var _curr-data/eax: (addr array byte) <- lookup *curr-data-ah
-        var curr-data/edx: (addr array byte) <- copy _curr-data
-        var x/eax: int <- copy xmin
-        x, y <- draw-text-wrapping-right-then-down screen, curr-data, xmin, ymin, xmax, ymax, x, y, 0xc/fg=trace-error, bg
-        y <- increment
+        y <- render-trace-line screen, curr, xmin, y, xmax, ymax, 0xc/fg=trace-error, bg
         copy-to already-hiding-lines?, 0/false
         break $render-trace:iter
       }
-      # otherwise ignore the rest
+      # display expanded lines
+      var display?/eax: boolean <- should-render? self, curr
+      {
+        compare display?, 0/false
+        break-if-=
+        y <- render-trace-line screen, curr, xmin, y, xmax, ymax, 9/fg=blue, bg
+        copy-to already-hiding-lines?, 0/false
+        break $render-trace:iter
+      }
+      # ignore the rest
       compare already-hiding-lines?, 0/false
       {
         break-if-!=
@@ -160,10 +193,113 @@ fn render-trace screen: (addr screen), _self: (addr trace), xmin: int, ymin: int
     loop
   }
   # prevent cursor from going too far down
-  clamp-cursor-to-bottom _self, y
+  clamp-cursor-to-bottom self, y
+  mark-lines-clean self
   return y
 }
 
+fn render-trace-line screen: (addr screen), _self: (addr trace-line), xmin: int, ymin: int, xmax: int, ymax: int, fg: int, bg: int -> _/ecx: int {
+  var self/esi: (addr trace-line) <- copy _self
+  var xsave/edx: int <- copy xmin
+  var y/ecx: int <- copy ymin
+  var label-ah/eax: (addr handle array byte) <- get self, label
+  var _label/eax: (addr array byte) <- lookup *label-ah
+  var label/ebx: (addr array byte) <- copy _label
+  var is-error?/eax: boolean <- string-equal? label, "error"
+  compare is-error?, 0/false
+  {
+    break-if-!=
+    var x/eax: int <- copy xsave
+    {
+      var depth/edx: (addr int) <- get self, depth
+      x, y <- draw-int32-decimal-wrapping-right-then-down screen, *depth, xmin, ymin, xmax, ymax, x, y, fg, bg
+      x, y <- draw-text-wrapping-right-then-down screen, " ", xmin, ymin, xmax, ymax, x, y, fg, bg
+      x, y <- draw-text-wrapping-right-then-down screen, label, xmin, ymin, xmax, ymax, x, y, fg, bg
+      x, y <- draw-text-wrapping-right-then-down screen, ": ", xmin, ymin, xmax, ymax, x, y, fg, bg
+    }
+    xsave <- copy x
+  }
+  var data-ah/eax: (addr handle array byte) <- get self, data
+  var _data/eax: (addr array byte) <- lookup *data-ah
+  var data/ebx: (addr array byte) <- copy _data
+  var x/eax: int <- copy xsave
+  x, y <- draw-text-wrapping-right-then-down screen, data, xmin, ymin, xmax, ymax, x, y, fg, bg
+  y <- increment
+  return y
+}
+
+fn should-render? _self: (addr trace), _line: (addr trace-line) -> _/eax: boolean {
+  var self/esi: (addr trace) <- copy _self
+  # if visible? is already cached, just return it
+  var dest/edx: (addr boolean) <- get self, recompute-visible?
+  compare *dest, 0/false
+  {
+    break-if-!=
+    var line/eax: (addr trace-line) <- copy _line
+    var result/eax: (addr boolean) <- get line, visible?
+    return *result
+  }
+  # recompute
+  var candidates-ah/eax: (addr handle array trace-line) <- get self, visible
+  var candidates/eax: (addr array trace-line) <- lookup *candidates-ah
+  var i/ecx: int <- copy 0
+  var len/edx: int <- length candidates
+  {
+    compare i, len
+    break-if->=
+    {
+      var curr-offset/ecx: (offset trace-line) <- compute-offset candidates, i
+      var curr/ecx: (addr trace-line) <- index candidates, curr-offset
+      var match?/eax: boolean <- trace-lines-equal? curr, _line
+      compare match?, 0/false
+      break-if-=
+      var line/eax: (addr trace-line) <- copy _line
+      var dest/eax: (addr boolean) <- get line, visible?
+      copy-to *dest, 1/true
+      return 1/true
+    }
+    i <- increment
+    loop
+  }
+  var line/eax: (addr trace-line) <- copy _line
+  var dest/eax: (addr boolean) <- get line, visible?
+  copy-to *dest, 0/false
+  return 0/false
+}
+
+# this is probably super-inefficient, string comparing every trace line
+# against every visible line on every render
+fn trace-lines-equal? _a: (addr trace-line), _b: (addr trace-line) -> _/eax: boolean {
+  var a/esi: (addr trace-line) <- copy _a
+  var b/edi: (addr trace-line) <- copy _b
+  var a-depth/ecx: (addr int) <- get a, depth
+  var b-depth/edx: (addr int) <- get b, depth
+  var benchmark/eax: int <- copy *b-depth
+  compare *a-depth, benchmark
+  {
+    break-if-=
+    return 0/false
+  }
+  var a-label-ah/eax: (addr handle array byte) <- get a, label
+  var _a-label/eax: (addr array byte) <- lookup *a-label-ah
+  var a-label/ecx: (addr array byte) <- copy _a-label
+  var b-label-ah/ebx: (addr handle array byte) <- get b, label
+  var b-label/eax: (addr array byte) <- lookup *b-label-ah
+  var label-match?/eax: boolean <- string-equal? a-label, b-label
+  {
+    compare label-match?, 0/false
+    break-if-!=
+    return 0/false
+  }
+  var a-data-ah/eax: (addr handle array byte) <- get a, data
+  var _a-data/eax: (addr array byte) <- lookup *a-data-ah
+  var a-data/ecx: (addr array byte) <- copy _a-data
+  var b-data-ah/ebx: (addr handle array byte) <- get b, data
+  var b-data/eax: (addr array byte) <- lookup *b-data-ah
+  var data-match?/eax: boolean <- string-equal? a-data, b-data
+  return data-match?
+}
+
 fn clamp-cursor-to-top _self: (addr trace), _y: int {
   var y/ecx: int <- copy _y
   var self/esi: (addr trace) <- copy _self
@@ -185,7 +321,7 @@ fn clamp-cursor-to-bottom _self: (addr trace), _y: int {
 fn test-render-trace-empty {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   # setup: screen
   var screen-on-stack: screen
   var screen/edi: (addr screen) <- address screen-on-stack
@@ -200,7 +336,7 @@ fn test-render-trace-empty {
 fn test-render-trace-collapsed-by-default {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   trace-text t, "l", "data"
   # setup: screen
   var screen-on-stack: screen
@@ -216,7 +352,7 @@ fn test-render-trace-collapsed-by-default {
 fn test-render-trace-error {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   error t, "error"
   # setup: screen
   var screen-on-stack: screen
@@ -232,7 +368,7 @@ fn test-render-trace-error {
 fn test-render-trace-error-at-start {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   error t, "error"
   trace-text t, "l", "data"
@@ -251,7 +387,7 @@ fn test-render-trace-error-at-start {
 fn test-render-trace-error-at-end {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   trace-text t, "l", "data"
   error t, "error"
@@ -270,7 +406,7 @@ fn test-render-trace-error-at-end {
 fn test-render-trace-error-in-the-middle {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   trace-text t, "l", "line 1"
   error t, "error"
@@ -291,7 +427,7 @@ fn test-render-trace-error-in-the-middle {
 fn test-render-trace-cursor-in-single-line {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   trace-text t, "l", "line 1"
   error t, "error"
@@ -326,6 +462,10 @@ fn render-trace-menu screen: (addr screen) {
   draw-text-rightward-from-cursor screen, " cursor up  ", width, 7/fg, 0/bg
   draw-text-rightward-from-cursor screen, " tab ", width, 0/fg, 3/bg=cyan
   draw-text-rightward-from-cursor screen, " move to sandbox  ", width, 7/fg, 0/bg
+  draw-text-rightward-from-cursor screen, " enter ", width, 0/fg, 7/bg=grey
+  draw-text-rightward-from-cursor screen, " expand  ", width, 7/fg, 0/bg
+  draw-text-rightward-from-cursor screen, " backspace ", width, 0/fg, 7/bg=grey
+  draw-text-rightward-from-cursor screen, " collapse  ", width, 7/fg, 0/bg
 }
 
 fn edit-trace _self: (addr trace), key: grapheme {
@@ -346,12 +486,90 @@ fn edit-trace _self: (addr trace), key: grapheme {
     decrement *cursor-y
     return
   }
+  # enter
+  {
+    compare key, 0xa/newline
+    break-if-!=
+    expand self
+    return
+  }
+}
+
+fn expand _self: (addr trace) {
+  var self/esi: (addr trace) <- copy _self
+  var trace-ah/eax: (addr handle array trace-line) <- get self, data
+  var _trace/eax: (addr array trace-line) <- lookup *trace-ah
+  var trace/edi: (addr array trace-line) <- copy _trace
+  var cursor-line-index-addr/ecx: (addr int) <- get self, cursor-line-index
+  var cursor-line-index/ecx: int <- copy *cursor-line-index-addr
+  var cursor-line-offset/eax: (offset trace-line) <- compute-offset trace, cursor-line-index
+  var cursor-line/edx: (addr trace-line) <- index trace, cursor-line-offset
+  var cursor-line-visible?/eax: (addr boolean) <- get cursor-line, visible?
+  compare *cursor-line-visible?, 0/false
+  var cursor-line-depth/ebx: (addr int) <- get cursor-line, depth
+  var target-depth/ebx: int <- copy *cursor-line-depth
+  # if cursor-line is already visible, increment target-depth
+  compare *cursor-line-visible?, 0/false
+  {
+    break-if-=
+    target-depth <- increment
+  }
+  # reveal the run of lines starting at cursor-line-index with depth target-depth
+  var i/ecx: int <- copy cursor-line-index
+  var max/edx: (addr int) <- get self, first-free
+  {
+    compare i, *max
+    break-if->=
+    var curr-line-offset/eax: (offset trace-line) <- compute-offset trace, i
+    var curr-line/edx: (addr trace-line) <- index trace, curr-line-offset
+    var curr-line-depth/eax: (addr int) <- get curr-line, depth
+    compare *curr-line-depth, target-depth
+    break-if-<
+    {
+      break-if-!=
+      var curr-line-visible?/eax: (addr boolean) <- get curr-line, visible?
+      copy-to *curr-line-visible?, 1/true
+      reveal-trace-line self, curr-line
+    }
+    i <- increment
+    loop
+  }
+}
+
+# the 'visible' array is not required to be in order
+# elements can also be deleted out of order
+# so it can have holes
+# however, lines in it always have visible? set
+# we'll use visible? being unset as a sign of emptiness
+fn reveal-trace-line _self: (addr trace), line: (addr trace-line) {
+  var self/esi: (addr trace) <- copy _self
+  var visible-ah/eax: (addr handle array trace-line) <- get self, visible
+  var visible/eax: (addr array trace-line) <- lookup *visible-ah
+  var i/ecx: int <- copy 0
+  var len/edx: int <- length visible
+  {
+    compare i, len
+    break-if->=
+    var curr-offset/edx: (offset trace-line) <- compute-offset visible, i
+    var curr/edx: (addr trace-line) <- index visible, curr-offset
+    var curr-visible?/eax: (addr boolean) <- get curr, visible?
+    compare *curr-visible?, 0/false
+    {
+      break-if-!=
+      # empty slot found
+      copy-object line, curr
+      return
+    }
+    i <- increment
+    loop
+  }
+  abort "too many visible lines; increase size of array trace.visible"
 }
 
 fn test-cursor-down-and-up-within-trace {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   trace-text t, "l", "line 1"
   error t, "error"
@@ -394,7 +612,7 @@ fn test-cursor-down-and-up-within-trace {
 fn test-cursor-down-past-bottom-of-trace {
   var t-storage: trace
   var t/esi: (addr trace) <- address t-storage
-  initialize-trace t, 0x10
+  initialize-trace t, 0x10, 0x10
   #
   trace-text t, "l", "line 1"
   error t, "error"
@@ -418,7 +636,7 @@ fn test-cursor-down-past-bottom-of-trace {
   edit-trace t, 4/ctrl-d
   edit-trace t, 4/ctrl-d
   edit-trace t, 4/ctrl-d
-  # hack: we do need to render to make this test pass; a sign that we're mixing state management with rendering
+  # hack: we do need to render to make this test pass; we're mixing state management with rendering
   var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0xa/xmax, 4/ymax, 1/show-cursor
   # cursor disappears past bottom
   check-screen-row screen,                                  0/y, "...   ", "F - test-cursor-down-past-bottom-of-trace/down-0"
@@ -438,3 +656,303 @@ fn test-cursor-down-past-bottom-of-trace {
   check-screen-row screen,                                  2/y, "...   ", "F - test-cursor-down-past-bottom-of-trace/up-2"
   check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "|||   ", "F - test-cursor-down-past-bottom-of-trace/up-2/cursor"
 }
+
+fn test-cursor-expand-within-trace {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-text t, "l", "line 2"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...         ", "F - test-cursor-expand-within-trace/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||         ", "F - test-cursor-expand-within-trace/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "            ", "F - test-cursor-expand-within-trace/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-within-trace/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1 ", "F - test-cursor-expand-within-trace/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "||||||||||| ", "F - test-cursor-expand-within-trace/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2 ", "F - test-cursor-expand-within-trace/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-within-trace/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "            ", "F - test-cursor-expand-within-trace/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "            ", "F - test-cursor-expand-within-trace/expand-2/cursor"
+}
+
+fn test-cursor-expand-skips-lower-depth {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-lower t
+  trace-text t, "l", "line 2"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...         ", "F - test-cursor-expand-skips-lower-depth/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||         ", "F - test-cursor-expand-skips-lower-depth/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "            ", "F - test-cursor-expand-skips-lower-depth/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-skips-lower-depth/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1 ", "F - test-cursor-expand-skips-lower-depth/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "||||||||||| ", "F - test-cursor-expand-skips-lower-depth/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "...         ", "F - test-cursor-expand-skips-lower-depth/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-skips-lower-depth/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "            ", "F - test-cursor-expand-skips-lower-depth/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "            ", "F - test-cursor-expand-skips-lower-depth/expand-2/cursor"
+}
+
+fn test-cursor-expand-continues-past-lower-depth {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-lower t
+  trace-text t, "l", "line 1.1"
+  trace-higher t
+  trace-text t, "l", "line 2"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...         ", "F - test-cursor-expand-continues-past-lower-depth/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||         ", "F - test-cursor-expand-continues-past-lower-depth/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "            ", "F - test-cursor-expand-continues-past-lower-depth/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-continues-past-lower-depth/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1 ", "F - test-cursor-expand-continues-past-lower-depth/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "||||||||||| ", "F - test-cursor-expand-continues-past-lower-depth/expand-0/cursor"
+  # TODO: might be too wasteful to show every place where lines are hidden
+  check-screen-row screen,                                  1/y, "...         ", "F - test-cursor-expand-continues-past-lower-depth/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "            ", "F - test-cursor-expand-continues-past-lower-depth/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 2 ", "F - test-cursor-expand-continues-past-lower-depth/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "            ", "F - test-cursor-expand-continues-past-lower-depth/expand-2/cursor"
+}
+
+fn test-cursor-expand-stops-at-higher-depth {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1.1"
+  trace-lower t
+  trace-text t, "l", "line 1.1.1"
+  trace-higher t
+  trace-text t, "l", "line 1.2"
+  trace-higher t
+  trace-text t, "l", "line 2"
+  trace-lower t
+  trace-text t, "l", "line 2.1"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 8/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 8/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...           ", "F - test-cursor-expand-continues-past-lower-depth/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||           ", "F - test-cursor-expand-continues-past-lower-depth/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 8/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1.1 ", "F - test-cursor-expand-continues-past-lower-depth/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "||||||||||||| ", "F - test-cursor-expand-continues-past-lower-depth/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "...           ", "F - test-cursor-expand-continues-past-lower-depth/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 1.2 ", "F - test-cursor-expand-continues-past-lower-depth/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/expand-2/cursor"
+  check-screen-row screen,                                  3/y, "...           ", "F - test-cursor-expand-continues-past-lower-depth/expand-3"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 3/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/expand-3/cursor"
+  check-screen-row screen,                                  4/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/expand-4"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 4/y, "              ", "F - test-cursor-expand-continues-past-lower-depth/expand-4/cursor"
+}
+
+fn test-cursor-expand-twice {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-lower t
+  trace-text t, "l", "line 1.1"
+  trace-higher t
+  trace-text t, "l", "line 2"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...           ", "F - test-cursor-expand-twice/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||           ", "F - test-cursor-expand-twice/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "              ", "F - test-cursor-expand-twice/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-expand-twice/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-expand-twice/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||||||||||   ", "F - test-cursor-expand-twice/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "...           ", "F - test-cursor-expand-twice/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-expand-twice/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 2   ", "F - test-cursor-expand-twice/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-expand-twice/expand-2/cursor"
+  # cursor down
+  edit-trace t, 4/ctrl-d
+  # hack: we need to render here to make this test pass; we're mixing state management with rendering
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-expand-twice/down-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-expand-twice/down-0/cursor"
+  check-screen-row screen,                                  1/y, "...           ", "F - test-cursor-expand-twice/down-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "|||           ", "F - test-cursor-expand-twice/down-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 2   ", "F - test-cursor-expand-twice/down-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-expand-twice/down-2/cursor"
+  # expand again
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-expand-twice/expand2-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-expand-twice/expand2-0/cursor"
+  check-screen-row screen,                                  1/y, "1 l: line 1.1 ", "F - test-cursor-expand-twice/expand2-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "||||||||||||| ", "F - test-cursor-expand-twice/expand2-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 2   ", "F - test-cursor-expand-twice/expand2-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-expand-twice/expand2-2/cursor"
+}
+
+fn test-cursor-refresh-cursor {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-text t, "l", "line 2"
+  trace-text t, "l", "line 3"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...           ", "F - test-cursor-refresh-cursor/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||           ", "F - test-cursor-refresh-cursor/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "              ", "F - test-cursor-refresh-cursor/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-refresh-cursor/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-refresh-cursor/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||||||||||   ", "F - test-cursor-refresh-cursor/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2   ", "F - test-cursor-refresh-cursor/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-refresh-cursor/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-refresh-cursor/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-refresh-cursor/expand-2/cursor"
+  # cursor down
+  edit-trace t, 4/ctrl-d
+  edit-trace t, 4/ctrl-d
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-refresh-cursor/down-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-refresh-cursor/down-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2   ", "F - test-cursor-refresh-cursor/down-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-refresh-cursor/down-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-refresh-cursor/down-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "|||||||||||   ", "F - test-cursor-refresh-cursor/down-2/cursor"
+  # recreate trace
+  clear-trace t
+  trace-text t, "l", "line 1"
+  trace-text t, "l", "line 2"
+  trace-text t, "l", "line 3"
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  # cursor remains unchanged
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-refresh-cursor/refresh-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-refresh-cursor/refresh-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2   ", "F - test-cursor-refresh-cursor/refresh-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-refresh-cursor/refresh-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-refresh-cursor/refresh-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "|||||||||||   ", "F - test-cursor-refresh-cursor/refresh-2/cursor"
+}
+
+fn test-cursor-preserve-cursor-on-edit-in-other-line {
+  var t-storage: trace
+  var t/esi: (addr trace) <- address t-storage
+  initialize-trace t, 0x10, 0x10
+  #
+  trace-text t, "l", "line 1"
+  trace-text t, "l", "line 2"
+  trace-text t, "l", "line 3"
+  # setup: screen
+  var screen-on-stack: screen
+  var screen/edi: (addr screen) <- address screen-on-stack
+  initialize-screen screen, 0x10/width, 4/height
+  #
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "...           ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/pre-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||           ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/pre-0/cursor"
+  check-screen-row screen,                                  1/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/pre-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/pre-1/cursor"
+  # expand
+  edit-trace t, 0xa/enter
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "|||||||||||   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/expand-2/cursor"
+  # cursor down
+  edit-trace t, 4/ctrl-d
+  edit-trace t, 4/ctrl-d
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  #
+  check-screen-row screen,                                  0/y, "0 l: line 1   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 2   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "|||||||||||   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/down-2/cursor"
+  # recreate trace with slightly different lines
+  clear-trace t
+  trace-text t, "l", "line 4"
+  trace-text t, "l", "line 5"
+  trace-text t, "l", "line 3"  # cursor line is unchanged
+  var y/ecx: int <- render-trace screen, t, 0/xmin, 0/ymin, 0x10/xmax, 4/ymax, 1/show-cursor
+  # cursor remains unchanged
+  check-screen-row screen,                                  0/y, "0 l: line 4   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-0"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 0/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-0/cursor"
+  check-screen-row screen,                                  1/y, "0 l: line 5   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-1"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 1/y, "              ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-1/cursor"
+  check-screen-row screen,                                  2/y, "0 l: line 3   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-2"
+  check-background-color-in-screen-row screen, 7/bg=cursor, 2/y, "|||||||||||   ", "F - test-cursor-preserve-cursor-on-edit-in-other-line/refresh-2/cursor"
+}