diff --git a/app.lua b/app.lua
index 1d4a72b..584b17b 100644
--- a/app.lua
+++ b/app.lua
@@ -361,6 +361,7 @@ function App.run_tests()
   for _,name in ipairs(sorted_names) do
     App.initialize_for_test()
 --?     print('=== '..name)
+--?     _G[name]()
     xpcall(_G[name], function(err) prepend_debug_info_to_test_failure(name, err) end)
   end
   -- clean up all test methods
diff --git a/edit.lua b/edit.lua
index 45aa879..3c7a5ad 100644
--- a/edit.lua
+++ b/edit.lua
@@ -43,8 +43,6 @@ function edit.initialize_state(top, left, right, font_height, line_height)  -- c
     old_cursor1 = nil,
     old_selection1 = nil,
     mousepress_shift = nil,
-    -- when selecting text, avoid recomputing some state on every single frame
-    recent_mouse = {},
 
     -- cursor coordinates in pixels
     cursor_x = 0,
@@ -103,20 +101,22 @@ function edit.draw(State)
   State.cursor_x = nil
   State.cursor_y = nil
   local y = State.top
+  local screen_bottom1 = {line=nil, pos=nil}
 --?   print('== draw')
   for line_index = State.screen_top1.line,#State.lines do
     local line = State.lines[line_index]
 --?     print('draw:', y, line_index, line)
     if y + State.line_height > App.screen.height then break end
-    State.screen_bottom1 = {line=line_index, pos=nil}
+    screen_bottom1.line = line_index
 --?     print('text.draw', y, line_index)
     local startpos = 1
     if line_index == State.screen_top1.line then
       startpos = State.screen_top1.pos
     end
-    y, State.screen_bottom1.pos = Text.draw(State, line_index, y, startpos)
+    y, screen_bottom1.pos = Text.draw(State, line_index, y, startpos)
 --?     print('=> y', y)
   end
+  State.screen_bottom1 = screen_bottom1
   if State.search_term then
     Text.draw_search_bar(State)
   end
@@ -146,7 +146,18 @@ end
 
 function edit.mouse_press(State, x,y, mouse_button)
   if State.search_term then return end
---?   print('press', State.cursor1.line)
+--?   print_and_log(('edit.mouse_press: cursor at %d,%d'):format(State.cursor1.line, State.cursor1.pos))
+  if y < State.top then
+    State.old_cursor1 = State.cursor1
+    State.old_selection1 = State.selection1
+    State.mousepress_shift = App.shift_down()
+    State.selection1 = {
+        line=State.screen_top1.line,
+        pos=State.screen_top1.pos,
+    }
+    return
+  end
+
   for line_index,line in ipairs(State.lines) do
     if Text.in_line(State, line_index, x,y) then
       -- delicate dance between cursor, selection and old cursor/selection
@@ -157,7 +168,8 @@ function edit.mouse_press(State, x,y, mouse_button)
       --    sets cursor
       --  press and hold to start a selection: sets selection on press, cursor on release
       --  press and hold, then press shift: ignore shift
-      --    i.e. mouse_released should never look at shift state
+      --    i.e. mouse_release should never look at shift state
+--?       print_and_log(('edit.mouse_press: in line %d'):format(line_index))
       State.old_cursor1 = State.cursor1
       State.old_selection1 = State.selection1
       State.mousepress_shift = App.shift_down()
@@ -165,23 +177,31 @@ function edit.mouse_press(State, x,y, mouse_button)
           line=line_index,
           pos=Text.to_pos_on_line(State, line_index, x, y),
       }
---?       print('selection', State.selection1.line, State.selection1.pos)
-      break
+      return
     end
   end
+
+  -- still here? click is below all screen lines
+  State.old_cursor1 = State.cursor1
+  State.old_selection1 = State.selection1
+  State.mousepress_shift = App.shift_down()
+  State.selection1 = {
+      line=State.screen_bottom1.line,
+      pos=Text.pos_at_end_of_screen_line(State, State.screen_bottom1),
+  }
 end
 
 function edit.mouse_release(State, x,y, mouse_button)
   if State.search_term then return end
---?   print('release', State.cursor1.line)
+--?   print_and_log(('edit.mouse_release(%d,%d): cursor at %d,%d'):format(x,y, State.cursor1.line, State.cursor1.pos))
   for line_index,line in ipairs(State.lines) do
     if Text.in_line(State, line_index, x,y) then
---?       print('reset selection')
+--?       print_and_log(('edit.mouse_release: in line %d'):format(line_index))
       State.cursor1 = {
           line=line_index,
           pos=Text.to_pos_on_line(State, line_index, x, y),
       }
---?       print('cursor', State.cursor1.line, State.cursor1.pos)
+--?       print_and_log(('edit.mouse_release: cursor now %d,%d'):format(State.cursor1.line, State.cursor1.pos))
       if State.mousepress_shift then
         if State.old_selection1.line == nil then
           State.selection1 = State.old_cursor1
@@ -196,7 +216,7 @@ function edit.mouse_release(State, x,y, mouse_button)
       break
     end
   end
---?   print('selection:', State.selection1.line, State.selection1.pos)
+--?   print_and_log(('edit.mouse_release: finally selection %s,%s cursor %d,%d'):format(tostring(State.selection1.line), tostring(State.selection1.pos), State.cursor1.line, State.cursor1.pos))
 end
 
 function edit.mouse_wheel_move(State, dx,dy)
diff --git a/run.lua b/run.lua
index 2aba0e9..4c8968a 100644
--- a/run.lua
+++ b/run.lua
@@ -48,6 +48,11 @@ function run.initialize(arg)
   end
 end
 
+function print_and_log(s)
+  print(s)
+  log(3, s)
+end
+
 function run.load_settings()
   love.graphics.setFont(love.graphics.newFont(Settings.font_height))
   -- determine default dimensions and flags
diff --git a/select.lua b/select.lua
index 0050895..a24b828 100644
--- a/select.lua
+++ b/select.lua
@@ -29,7 +29,6 @@ function Text.clip_selection(State, line_index, apos, bpos)
   -- compare bounds more carefully (start inclusive, end exclusive)
   local a_ge = Text.le1({line=minl, pos=minp}, {line=line_index, pos=apos})
   local b_lt = Text.lt1({line=line_index, pos=bpos}, {line=maxl, pos=maxp})
---?   print(minl,line_index,maxl, '--', minp,apos,bpos,maxp, '--', a_ge,b_lt)
   if a_ge and b_lt then
     -- fully contained
     return apos,bpos
@@ -60,7 +59,6 @@ function Text.draw_highlight(State, line, x,y, pos, lo,hi)
       local before = line.data:sub(pos_offset, lo_offset-1)
       lo_px = App.width(before)
     end
---?     print(lo,pos,hi, '--', lo_offset,pos_offset,hi_offset, '--', lo_px)
     local s = line.data:sub(lo_offset, hi_offset-1)
     App.color(Highlight_color)
     love.graphics.rectangle('fill', x+lo_px,y, App.width(s),State.line_height)
@@ -69,27 +67,17 @@ function Text.draw_highlight(State, line, x,y, pos, lo,hi)
   end
 end
 
--- inefficient for some reason, so don't do it on every frame
 function Text.mouse_pos(State)
-  local time = love.timer.getTime()
-  if State.recent_mouse.time and State.recent_mouse.time > time-0.1 then
-    return State.recent_mouse.line, State.recent_mouse.pos
+  local x,y = App.mouse_x(), App.mouse_y()
+  if y < State.line_cache[State.screen_top1.line].starty then
+    return State.screen_top1.line, State.screen_top1.pos
   end
-  State.recent_mouse.time = time
-  local line,pos = Text.to_pos(State, App.mouse_x(), App.mouse_y())
-  if line then
-    State.recent_mouse.line = line
-    State.recent_mouse.pos = pos
-  end
-  return State.recent_mouse.line, State.recent_mouse.pos
-end
-
-function Text.to_pos(State, x,y)
   for line_index,line in ipairs(State.lines) do
     if Text.in_line(State, line_index, x,y) then
       return line_index, Text.to_pos_on_line(State, line_index, x,y)
     end
   end
+  return State.screen_bottom1.line, Text.pos_at_end_of_screen_line(State, State.screen_bottom1)
 end
 
 function Text.cut_selection(State)
diff --git a/source.lua b/source.lua
index de120c0..6f5ff0a 100644
--- a/source.lua
+++ b/source.lua
@@ -113,6 +113,11 @@ function source.initialize_edit_side()
   end
 end
 
+function print_and_log(s)
+  print(s)
+  log(3, s)
+end
+
 function source.load_settings()
   local settings = Settings.source
   love.graphics.setFont(love.graphics.newFont(settings.font_height))
diff --git a/source_edit.lua b/source_edit.lua
index 58b1ccc..565b989 100644
--- a/source_edit.lua
+++ b/source_edit.lua
@@ -76,8 +76,6 @@ function edit.initialize_state(top, left, right, font_height, line_height)  -- c
     old_cursor1 = nil,
     old_selection1 = nil,
     mousepress_shift = nil,
-    -- when selecting text, avoid recomputing some state on every single frame
-    recent_mouse = {},
 
     -- cursor coordinates in pixels
     cursor_x = 0,
@@ -155,12 +153,13 @@ function edit.draw(State, hide_cursor)
   State.cursor_x = nil
   State.cursor_y = nil
   local y = State.top
+  local screen_bottom1 = {line=nil, pos=nil}
 --?   print('== draw')
   for line_index = State.screen_top1.line,#State.lines do
     local line = State.lines[line_index]
 --?     print('draw:', y, line_index, line)
     if y + State.line_height > App.screen.height then break end
-    State.screen_bottom1 = {line=line_index, pos=nil}
+    screen_bottom1.line = line_index
     if line.mode == 'text' then
 --?       print('text.draw', y, line_index)
       local startpos = 1
@@ -183,7 +182,7 @@ function edit.draw(State, hide_cursor)
                      end,
         })
       end
-      y, State.screen_bottom1.pos = Text.draw(State, line_index, y, startpos, hide_cursor)
+      y, screen_bottom1.pos = Text.draw(State, line_index, y, startpos, hide_cursor)
 --?       print('=> y', y)
     elseif line.mode == 'drawing' then
       y = y+Drawing_padding_top
@@ -194,6 +193,7 @@ function edit.draw(State, hide_cursor)
       assert(false)
     end
   end
+  State.screen_bottom1 = screen_bottom1
   if State.search_term then
     Text.draw_search_bar(State)
   end
@@ -224,12 +224,23 @@ end
 
 function edit.mouse_press(State, x,y, mouse_button)
   if State.search_term then return end
---?   print('press', State.cursor1.line)
+--?   print_and_log(('edit.mouse_press: cursor at %d,%d'):format(State.cursor1.line, State.cursor1.pos))
   if mouse_press_consumed_by_any_button_handler(State, x,y, mouse_button) then
     -- press on a button and it returned 'true' to short-circuit
     return
   end
 
+  if y < State.top then
+    State.old_cursor1 = State.cursor1
+    State.old_selection1 = State.selection1
+    State.mousepress_shift = App.shift_down()
+    State.selection1 = {
+        line=State.screen_top1.line,
+        pos=State.screen_top1.pos,
+    }
+    return
+  end
+
   for line_index,line in ipairs(State.lines) do
     if line.mode == 'text' then
       if Text.in_line(State, line_index, x,y) then
@@ -242,6 +253,7 @@ function edit.mouse_press(State, x,y, mouse_button)
         --  press and hold to start a selection: sets selection on press, cursor on release
         --  press and hold, then press shift: ignore shift
         --    i.e. mouse_release should never look at shift state
+--?         print_and_log(('edit.mouse_press: in line %d'):format(line_index))
         State.old_cursor1 = State.cursor1
         State.old_selection1 = State.selection1
         State.mousepress_shift = App.shift_down()
@@ -249,8 +261,7 @@ function edit.mouse_press(State, x,y, mouse_button)
             line=line_index,
             pos=Text.to_pos_on_line(State, line_index, x, y),
         }
---?         print('selection', State.selection1.line, State.selection1.pos)
-        break
+        return
       end
     elseif line.mode == 'drawing' then
       local line_cache = State.line_cache[line_index]
@@ -259,15 +270,24 @@ function edit.mouse_press(State, x,y, mouse_button)
         State.lines.current_drawing = line
         Drawing.before = snapshot(State, line_index)
         Drawing.mouse_press(State, line_index, x,y, mouse_button)
-        break
+        return
       end
     end
   end
+
+  -- still here? click is below all screen lines
+  State.old_cursor1 = State.cursor1
+  State.old_selection1 = State.selection1
+  State.mousepress_shift = App.shift_down()
+  State.selection1 = {
+      line=State.screen_bottom1.line,
+      pos=Text.pos_at_end_of_screen_line(State, State.screen_bottom1),
+  }
 end
 
 function edit.mouse_release(State, x,y, mouse_button)
   if State.search_term then return end
---?   print('release', State.cursor1.line)
+--?   print_and_log(('edit.mouse_release: cursor at %d,%d'):format(State.cursor1.line, State.cursor1.pos))
   if State.lines.current_drawing then
     Drawing.mouse_release(State, x,y, mouse_button)
     schedule_save(State)
@@ -276,15 +296,16 @@ function edit.mouse_release(State, x,y, mouse_button)
       Drawing.before = nil
     end
   else
+--?     print_and_log('edit.mouse_release: no current drawing')
     for line_index,line in ipairs(State.lines) do
       if line.mode == 'text' then
         if Text.in_line(State, line_index, x,y) then
---?           print('reset selection')
+--?           print_and_log(('edit.mouse_release: in line %d'):format(line_index))
           State.cursor1 = {
               line=line_index,
               pos=Text.to_pos_on_line(State, line_index, x, y),
           }
---?           print('cursor', State.cursor1.line, State.cursor1.pos)
+--?           print_and_log(('edit.mouse_release: cursor now %d,%d'):format(State.cursor1.line, State.cursor1.pos))
           if State.mousepress_shift then
             if State.old_selection1.line == nil then
               State.selection1 = State.old_cursor1
@@ -300,7 +321,7 @@ function edit.mouse_release(State, x,y, mouse_button)
         end
       end
     end
---?     print('selection:', State.selection1.line, State.selection1.pos)
+--?     print_and_log(('edit.mouse_release: finally selection %s,%s cursor %d,%d'):format(tostring(State.selection1.line), tostring(State.selection1.pos), State.cursor1.line, State.cursor1.pos))
   end
 end
 
diff --git a/source_select.lua b/source_select.lua
index 2298513..9b2a278 100644
--- a/source_select.lua
+++ b/source_select.lua
@@ -1,6 +1,4 @@
 -- helpers for selecting portions of text
--- To keep things simple, we'll ignore the B side when selections start on the
--- A side, and stick to within a single B side selections start in.
 
 -- Return any intersection of the region from State.selection1 to State.cursor1 (or
 -- current mouse, if mouse is pressed; or recent mouse if mouse is pressed and
@@ -31,7 +29,6 @@ function Text.clip_selection(State, line_index, apos, bpos)
   -- compare bounds more carefully (start inclusive, end exclusive)
   local a_ge = Text.le1({line=minl, pos=minp}, {line=line_index, pos=apos})
   local b_lt = Text.lt1({line=line_index, pos=bpos}, {line=maxl, pos=maxp})
---?   print(minl,line_index,maxl, '--', minp,apos,bpos,maxp, '--', a_ge,b_lt)
   if a_ge and b_lt then
     -- fully contained
     return apos,bpos
@@ -62,7 +59,6 @@ function Text.draw_highlight(State, line, x,y, pos, lo,hi)
       local before = line.data:sub(pos_offset, lo_offset-1)
       lo_px = App.width(before)
     end
---?     print(lo,pos,hi, '--', lo_offset,pos_offset,hi_offset, '--', lo_px)
     local s = line.data:sub(lo_offset, hi_offset-1)
     App.color(Highlight_color)
     love.graphics.rectangle('fill', x+lo_px,y, App.width(s),State.line_height)
@@ -71,22 +67,11 @@ function Text.draw_highlight(State, line, x,y, pos, lo,hi)
   end
 end
 
--- inefficient for some reason, so don't do it on every frame
 function Text.mouse_pos(State)
-  local time = love.timer.getTime()
-  if State.recent_mouse.time and State.recent_mouse.time > time-0.1 then
-    return State.recent_mouse.line, State.recent_mouse.pos
+  local x,y = App.mouse_x(), App.mouse_y()
+  if y < State.line_cache[State.screen_top1.line].starty then
+    return State.screen_top1.line, State.screen_top1.pos
   end
-  State.recent_mouse.time = time
-  local line,pos = Text.to_pos(State, App.mouse_x(), App.mouse_y())
-  if line then
-    State.recent_mouse.line = line
-    State.recent_mouse.pos = pos
-  end
-  return State.recent_mouse.line, State.recent_mouse.pos
-end
-
-function Text.to_pos(State, x,y)
   for line_index,line in ipairs(State.lines) do
     if line.mode == 'text' then
       if Text.in_line(State, line_index, x,y) then
@@ -94,6 +79,7 @@ function Text.to_pos(State, x,y)
       end
     end
   end
+  return State.screen_bottom1.line, Text.pos_at_end_of_screen_line(State, State.screen_bottom1)
 end
 
 function Text.cut_selection(State)
diff --git a/source_text.lua b/source_text.lua
index 290d982..1a42310 100644
--- a/source_text.lua
+++ b/source_text.lua
@@ -650,6 +650,7 @@ function Text.right_without_scroll(State)
   end
 end
 
+-- result: pos, index of screen line
 function Text.pos_at_start_of_screen_line(State, loc1)
   Text.populate_screen_line_starting_pos(State, loc1.line)
   local line_cache = State.line_cache[loc1.line]
@@ -662,6 +663,20 @@ function Text.pos_at_start_of_screen_line(State, loc1)
   assert(false)
 end
 
+function Text.pos_at_end_of_screen_line(State, loc1)
+  Text.populate_screen_line_starting_pos(State, loc1.line)
+  local line_cache = State.line_cache[loc1.line]
+  local most_recent_final_pos = utf8.len(State.lines[loc1.line].data)+1
+  for i=#line_cache.screen_line_starting_pos,1,-1 do
+    local spos = line_cache.screen_line_starting_pos[i]
+    if spos <= loc1.pos then
+      return most_recent_final_pos
+    end
+    most_recent_final_pos = spos-1
+  end
+  assert(false)
+end
+
 function Text.cursor_at_final_screen_line(State)
   Text.populate_screen_line_starting_pos(State, State.cursor1.line)
   local screen_lines = State.line_cache[State.cursor1.line].screen_line_starting_pos
diff --git a/source_text_tests.lua b/source_text_tests.lua
index 11e7613..c2e054a 100644
--- a/source_text_tests.lua
+++ b/source_text_tests.lua
@@ -1,4 +1,6 @@
 -- major tests for text editing flows
+-- Arguably this should be called source_edit_tests.lua,
+-- but that would mess up the git blame at this point.
 
 function test_initial_state()
   App.screen.init{width=120, height=60}
@@ -828,6 +830,67 @@ function test_select_text_using_mouse()
   check_eq(Editor_state.cursor1.pos, 4, 'cursor:pos')
 end
 
+function test_select_text_using_mouse_starting_above_text()
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc', 'def', 'xyz'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  Editor_state.selection1 = {}
+  edit.draw(Editor_state)  -- populate line_cache.starty for each line Editor_state.line_cache
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, Editor_state.left+8,5, 1)
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 1, 'selection:pos')
+end
+
+function test_select_text_using_mouse_starting_above_text_wrapping_line()
+  -- first screen line starts in the middle of a line
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc', 'defgh', 'xyz'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=2, pos=5}
+  Editor_state.screen_top1 = {line=2, pos=3}
+  Editor_state.screen_bottom1 = {}
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, Editor_state.left+8,5, 1)
+  -- selection is at screen top
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 2, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 3, 'selection:pos')
+end
+
+function test_select_text_using_mouse_starting_below_text()
+  -- I'd like to test what happens when a mouse click is below some page of
+  -- text, potentially even in the middle of a line.
+  -- However, it's brittle to set up a text line boundary just right.
+  -- So I'm going to just check things below the bottom of the final line of
+  -- text when it's in the middle of the screen.
+  -- final screen line ends in the middle of screen
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abcde'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  edit.draw(Editor_state)
+  local y = Editor_state.top
+  App.screen.check(y, 'ab', 'baseline:screen:1')
+  y = y + Editor_state.line_height
+  App.screen.check(y, 'cde', 'baseline:screen:2')
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, 5,App.screen.height-5, 1)
+  -- selection is past bottom-most text in screen
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 6, 'selection:pos')
+end
+
 function test_select_text_using_mouse_and_shift()
   App.screen.init{width=50, height=60}
   Editor_state = edit.initialize_test_state()
@@ -882,6 +945,28 @@ function test_select_text_repeatedly_using_mouse_and_shift()
   check_eq(Editor_state.cursor1.pos, 2, 'cursor:pos')
 end
 
+function test_select_all_text()
+  -- display a single line of text
+  App.screen.init{width=75, height=80}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc def'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  edit.draw(Editor_state)
+  -- select all
+  App.fake_key_press('lctrl')
+  edit.run_after_keychord(Editor_state, 'C-a')
+  App.fake_key_release('lctrl')
+  edit.key_release(Editor_state, 'lctrl')
+  -- selection
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 1, 'selection:pos')
+  check_eq(Editor_state.cursor1.line, 1, 'cursor:line')
+  check_eq(Editor_state.cursor1.pos, 8, 'cursor:pos')
+end
+
 function test_cut_without_selection()
   -- display a few lines
   App.screen.init{width=Editor_state.left+30, height=60}
diff --git a/text.lua b/text.lua
index 43a758d..a51ca4c 100644
--- a/text.lua
+++ b/text.lua
@@ -549,6 +549,7 @@ function Text.right_without_scroll(State)
   end
 end
 
+-- result: pos, index of screen line
 function Text.pos_at_start_of_screen_line(State, loc1)
   Text.populate_screen_line_starting_pos(State, loc1.line)
   local line_cache = State.line_cache[loc1.line]
@@ -561,6 +562,20 @@ function Text.pos_at_start_of_screen_line(State, loc1)
   assert(false)
 end
 
+function Text.pos_at_end_of_screen_line(State, loc1)
+  Text.populate_screen_line_starting_pos(State, loc1.line)
+  local line_cache = State.line_cache[loc1.line]
+  local most_recent_final_pos = utf8.len(State.lines[loc1.line].data)+1
+  for i=#line_cache.screen_line_starting_pos,1,-1 do
+    local spos = line_cache.screen_line_starting_pos[i]
+    if spos <= loc1.pos then
+      return most_recent_final_pos
+    end
+    most_recent_final_pos = spos-1
+  end
+  assert(false)
+end
+
 function Text.cursor_at_final_screen_line(State)
   Text.populate_screen_line_starting_pos(State, State.cursor1.line)
   local screen_lines = State.line_cache[State.cursor1.line].screen_line_starting_pos
diff --git a/text_tests b/text_tests
new file mode 100644
index 0000000..d571368
--- /dev/null
+++ b/text_tests
@@ -0,0 +1,118 @@
+== Summary of tests for the text editor
+
+This doesn't include all tests and might not change between forks, but it's
+intended to be the "timeless core" of a text editor widget that shouldn't
+change across forks.
+
+# basic
+initial state
+draw text
+draw wrapping text
+draw word wrapping text
+draw text wrapping within word
+draw wrapping text containing non ascii
+
+# mouse
+click moves cursor
+click to left of line
+click takes margins into account
+click on empty line
+click on wrapping line
+click on wrapping line takes margins into account
+click on wrapping line
+click on wrapping line rendered from partway at top of screen
+click past end of wrapping line
+click past end of wrapping line containing non ascii
+click past end of word wrapping line
+
+# cursor movement
+move left
+  move left to previous line
+move right
+  move right to next line
+
+move to start of word
+  move to start of previous word
+  move to start of word on previous line
+move past end of word
+  move past end of word on next line
+skip to previous word
+  skip past tab to previous word
+  skip multiple spaces to previous word
+skip to next word
+  skip past tab to next word
+  skip multiple spaces to next word
+
+# mutating text
+insert first character
+edit wrapping text
+insert newline
+insert newline at start of line
+insert from clipboard
+backspace from start of final line
+backspace past line boundary
+backspace over selection
+backspace over selection reverse
+backspace over multiple lines
+backspace to end of line
+backspace to start of line
+
+# scroll
+pagedown
+pagedown often shows start of wrapping line
+pagedown can start from middle of long wrapping line
+pagedown never moves up
+down arrow moves cursor
+down arrow scrolls down by one line
+down arrow scrolls down by one screen line
+down arrow scrolls down by one screen line after splitting within word
+pagedown followed by down arrow does not scroll screen up
+up arrow moves cursor
+up arrow scrolls up by one line
+up arrow scrolls up by one screen line
+up arrow scrolls up to final screen line
+up arrow scrolls up to empty line
+pageup
+pageup scrolls up by screen line
+pageup scrolls up from middle screen line
+enter on bottom line scrolls down
+enter on final line avoids scrolling down when not at bottom
+inserting text on final line avoids scrolling down when not at bottom
+typing on bottom line scrolls down
+left arrow scrolls up in wrapped line
+right arrow scrolls down in wrapped line
+home scrolls up in wrapped line
+end scrolls down in wrapped line
+position cursor on recently edited wrapping line
+backspace can scroll up
+backspace can scroll up screen line
+
+# selection
+select text using shift and cursor movement operations
+select text using mouse
+  clicking to left of a line = start of line
+  clicking to right of a line = end of line
+  clicking above topmost line = top of screen
+  clicking below bottom-most line = bottom of screen
+select text using mouse and shift
+select text repeatedly using mouse and shift
+cursor movement without shift resets selection
+mouse click without shift resets selection
+edit deletes selection
+edit with shift key deletes selection
+deleting selection may scroll
+copy does not reset selection
+cut
+cut without selection
+paste replaces selection
+
+# search
+search
+search upwards
+search wrap
+search wrap upwards
+
+# undo
+undo insert text
+undo delete text
+undo restores selection
diff --git a/text_tests.lua b/text_tests.lua
index c5d689d..ccbc9be 100644
--- a/text_tests.lua
+++ b/text_tests.lua
@@ -1,4 +1,6 @@
 -- major tests for text editing flows
+-- Arguably this should be called edit_tests.lua,
+-- but that would mess up the git blame at this point.
 
 function test_initial_state()
   App.screen.init{width=120, height=60}
@@ -802,6 +804,67 @@ function test_select_text_using_mouse()
   check_eq(Editor_state.cursor1.pos, 4, 'cursor:pos')
 end
 
+function test_select_text_using_mouse_starting_above_text()
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc', 'def', 'xyz'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  Editor_state.selection1 = {}
+  edit.draw(Editor_state)  -- populate line_cache.starty for each line Editor_state.line_cache
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, Editor_state.left+8,5, 1)
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 1, 'selection:pos')
+end
+
+function test_select_text_using_mouse_starting_above_text_wrapping_line()
+  -- first screen line starts in the middle of a line
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc', 'defgh', 'xyz'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=2, pos=5}
+  Editor_state.screen_top1 = {line=2, pos=3}
+  Editor_state.screen_bottom1 = {}
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, Editor_state.left+8,5, 1)
+  -- selection is at screen top
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 2, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 3, 'selection:pos')
+end
+
+function test_select_text_using_mouse_starting_below_text()
+  -- I'd like to test what happens when a mouse click is below some page of
+  -- text, potentially even in the middle of a line.
+  -- However, it's brittle to set up a text line boundary just right.
+  -- So I'm going to just check things below the bottom of the final line of
+  -- text when it's in the middle of the screen.
+  -- final screen line ends in the middle of screen
+  App.screen.init{width=50, height=60}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abcde'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  edit.draw(Editor_state)
+  local y = Editor_state.top
+  App.screen.check(y, 'ab', 'baseline:screen:1')
+  y = y + Editor_state.line_height
+  App.screen.check(y, 'cde', 'baseline:screen:2')
+  -- press mouse above first line of text
+  edit.run_after_mouse_press(Editor_state, 5,App.screen.height-5, 1)
+  -- selection is past bottom-most text in screen
+  check(Editor_state.selection1.line ~= nil, 'selection:line-not-nil')
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 6, 'selection:pos')
+end
+
 function test_select_text_using_mouse_and_shift()
   App.screen.init{width=50, height=60}
   Editor_state = edit.initialize_test_state()
@@ -856,6 +919,28 @@ function test_select_text_repeatedly_using_mouse_and_shift()
   check_eq(Editor_state.cursor1.pos, 2, 'cursor:pos')
 end
 
+function test_select_all_text()
+  -- display a single line of text
+  App.screen.init{width=75, height=80}
+  Editor_state = edit.initialize_test_state()
+  Editor_state.lines = load_array{'abc def'}
+  Text.redraw_all(Editor_state)
+  Editor_state.cursor1 = {line=1, pos=1}
+  Editor_state.screen_top1 = {line=1, pos=1}
+  Editor_state.screen_bottom1 = {}
+  edit.draw(Editor_state)
+  -- select all
+  App.fake_key_press('lctrl')
+  edit.run_after_keychord(Editor_state, 'C-a')
+  App.fake_key_release('lctrl')
+  edit.key_release(Editor_state, 'lctrl')
+  -- selection
+  check_eq(Editor_state.selection1.line, 1, 'selection:line')
+  check_eq(Editor_state.selection1.pos, 1, 'selection:pos')
+  check_eq(Editor_state.cursor1.line, 1, 'cursor:line')
+  check_eq(Editor_state.cursor1.pos, 8, 'cursor:pos')
+end
+
 function test_cut_without_selection()
   -- display a few lines
   App.screen.init{width=Editor_state.left+30, height=60}