Fix folder path truncation in filemanager with utf8 chars (#3599)

frontend/apps/filemanager/filemanager.lua

@@ -48,13 +48,12 @@ end
 local function truncatePath(text)
     local screen_width = Screen:getWidth()
     local face = Font:getFace("xx_smallinfofont")
-    local reversed_text = string.reverse(text)
-    local txt_width = RenderText:sizeUtf8Text(0, screen_width, face, reversed_text, nil, false).x
+    -- we want to truncate text on the left, so work with the reverse of text (which is fine as we don't use kerning)
+    local reversed_text = require("util").utf8Reverse(text)
+    local txt_width = RenderText:sizeUtf8Text(0, screen_width, face, reversed_text, false, false).x
     if  screen_width - 2 * Size.padding.small < txt_width then
-        reversed_text = RenderText:truncateTextByWidth(reversed_text, face, screen_width - 2 * Size.padding.small, nil, false)
-        -- string.reverse() breaks "…" so we delete it
-        reversed_text = reversed_text:sub(1, -5)
-        text = "…" .. string.reverse(reversed_text)
+        reversed_text = RenderText:truncateTextByWidth(reversed_text, face, screen_width - 2 * Size.padding.small, false, false)
+        text = require("util").utf8Reverse(reversed_text)
     end
     return text
 end

frontend/util.lua

@@ -129,6 +129,13 @@ function util.lastIndexOf(string, ch)
     if i == nil then return -1 else return i - 1 end
 end
 
+--- Reverse the individual greater-than-single-byte characters
+-- @string string to reverse
+-- Taken from https://github.com/blitmap/lua-utf8-simple#utf8reverses
+function util.utf8Reverse(text)
+    text = text:gsub('[%z\1-\127\194-\244][\128-\191]*', function (c) return #c > 1 and c:reverse() end)
+    return text:reverse()
+end
 
 --- Splits string into a list of UTF-8 characters.
 ---- @string text the string to be split.