fix scrollbar spacing issue

testing/__tests__/scrollbar_placement_test.lua

@@ -0,0 +1,176 @@
+package.path = package.path .. ";./?.lua;./modules/?.lua"
+local originalSearchers = package.searchers or package.loaders
+table.insert(originalSearchers, 2, function(modname)
+  if modname:match("^FlexLove%.modules%.") then
+    local moduleName = modname:gsub("^FlexLove%.modules%.", "")
+    return function()
+      return require("modules." .. moduleName)
+    end
+  end
+end)
+require("testing.loveStub")
+local luaunit = require("testing.luaunit")
+local FlexLove = require("FlexLove")
+
+FlexLove.init()
+
+TestScrollbarPlacement = {}
+
+function TestScrollbarPlacement:setUp()
+  FlexLove.setMode("retained")
+end
+
+function TestScrollbarPlacement:test_reserve_space_with_percentage_height_children()
+  -- Test case from user: horizontal scroll container with 100% height children
+  -- Should NOT cause vertical overflow
+  local container = FlexLove.new({
+    width = 200,
+    height = 200,
+    positioning = "flex",
+    flexDirection = "horizontal",
+    overflow = "scroll", -- Always shows scrollbars
+    scrollbarPlacement = "reserve-space",
+  })
+
+  local child1 = FlexLove.new({
+    width = 100,
+    height = "100%",
+    parent = container,
+  })
+
+  -- Trigger layout
+  container:layoutChildren()
+  container._scrollManager:detectOverflow(container)
+
+  local overflowX, overflowY = container._scrollManager:hasOverflow()
+
+  -- Child height should be reduced to account for horizontal scrollbar
+  -- Default scrollbar is 12px + 2px padding on each side = 16px
+  -- So child height should be 200 - 16 = 184px
+  luaunit.assertEquals(child1.height, 184)
+
+  -- Should have horizontal overflow (scroll mode), but NOT vertical
+  luaunit.assertFalse(overflowY, "Should not have vertical overflow with 100% height child")
+end
+
+function TestScrollbarPlacement:test_reserve_space_with_percentage_width_children()
+  -- Vertical scroll container with 100% width children
+  local container = FlexLove.new({
+    width = 200,
+    height = 200,
+    positioning = "flex",
+    flexDirection = "column",
+    overflow = "scroll", -- Always shows scrollbars
+    scrollbarPlacement = "reserve-space",
+  })
+
+  local child1 = FlexLove.new({
+    width = "100%",
+    height = 100,
+    parent = container,
+  })
+
+  -- Trigger layout
+  container:layoutChildren()
+  container._scrollManager:detectOverflow(container)
+
+  local overflowX, overflowY = container._scrollManager:hasOverflow()
+
+  -- Child width should be reduced to account for vertical scrollbar
+  -- Default scrollbar is 12px + 2px padding on each side = 16px
+  -- So child width should be 200 - 16 = 184px
+  luaunit.assertEquals(child1.width, 184)
+
+  -- Should have vertical overflow (scroll mode), but NOT horizontal
+  luaunit.assertFalse(overflowX, "Should not have horizontal overflow with 100% width child")
+end
+
+function TestScrollbarPlacement:test_overlay_mode_no_size_adjustment()
+  -- Overlay mode should NOT adjust child sizes
+  local container = FlexLove.new({
+    width = 200,
+    height = 200,
+    positioning = "flex",
+    flexDirection = "horizontal",
+    overflow = "scroll",
+    scrollbarPlacement = "overlay",
+  })
+
+  local child1 = FlexLove.new({
+    width = 100,
+    height = "100%",
+    parent = container,
+  })
+
+  -- Trigger layout
+  container:layoutChildren()
+
+  -- Child height should be full 200px (no reduction for scrollbar)
+  luaunit.assertEquals(child1.height, 200)
+end
+
+function TestScrollbarPlacement:test_auto_overflow_reserves_space_only_when_needed()
+  -- With overflow="auto" and reserve-space mode, space is reserved preemptively
+  -- But scrollbars should only be visible when content actually overflows
+  local container = FlexLove.new({
+    width = 200,
+    height = 200,
+    positioning = "flex",
+    flexDirection = "column",
+    overflow = "auto",
+    scrollbarPlacement = "reserve-space",
+  })
+
+  -- Child that doesn't cause overflow
+  local child1 = FlexLove.new({
+    width = "100%",
+    height = 100,
+    parent = container,
+  })
+
+  -- Trigger layout and overflow detection
+  container:layoutChildren()
+  container._scrollManager:detectOverflow(container)
+
+  local overflowX, overflowY = container._scrollManager:hasOverflow()
+
+  -- No overflow detected since content (100px) < available height (184px after scrollbar reservation)
+  luaunit.assertFalse(overflowY, "Should not have overflow")
+  -- Space is reserved preemptively with overflow="auto" to avoid layout shifts
+  luaunit.assertEquals(child1.width, 184, "Space should be reserved preemptively with auto mode")
+end
+
+function TestScrollbarPlacement:test_vertical_overflow_detected_with_reserved_space()
+  -- Test that overflow is properly detected when using reserve-space
+  local container = FlexLove.new({
+    width = 200,
+    height = 200,
+    positioning = "flex",
+    flexDirection = "column",
+    overflow = "auto",
+    scrollbarPlacement = "reserve-space",
+  })
+
+  -- Child that WILL cause overflow
+  local child1 = FlexLove.new({
+    width = "100%",
+    height = 300,
+    parent = container,
+  })
+
+  -- Trigger layout and overflow detection
+  container:layoutChildren()
+  container._scrollManager:detectOverflow(container)
+
+  local overflowX, overflowY = container._scrollManager:hasOverflow()
+
+  -- Should detect vertical overflow
+  luaunit.assertTrue(overflowY, "Should detect vertical overflow")
+
+  -- Child width should be reduced for vertical scrollbar
+  luaunit.assertEquals(child1.width, 184, "Child width should be reduced for scrollbar")
+end
+
+if not _G.RUNNING_ALL_TESTS then
+  os.exit(luaunit.LuaUnit.run())
+end