textbox: Honor per-screen DPI

This makes the textbox' :draw() and :fit() callbacks use the DPI that is
specified in the given drawing context. With this, the textbox now scales
correctly if different screens have different DPI values.

Idea originally from Daniel.

Closes https://github.com/awesomeWM/awesome/pull/457.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/textbox.lua

@@ -20,16 +20,21 @@ local error = error
 local textbox = { mt = {} }
 
 --- Setup a pango layout for the given textbox and cairo context
-local function setup_layout(box, width, height)
+local function setup_layout(box, width, height, dpi)
     local layout = box._layout
     layout.width = Pango.units_from_double(width)
     layout.height = Pango.units_from_double(height)
+    if box.dpi ~= dpi then
+        box.dpi = dpi
+        box._ctx:set_resolution(dpi)
+        box._layout:context_changed()
+    end
 end
 
 --- Draw the given textbox on the given cairo context in the given geometry
 function textbox:draw(context, cr, width, height)
+    setup_layout(self, width, height, context.dpi)
     cr:update_layout(self._layout)
-    setup_layout(self, width, height)
     local ink, logical = self._layout:get_pixel_extents()
     local offset = 0
     if self._valign == "center" then
@@ -43,7 +48,7 @@ end
 
 --- Fit the given textbox
 function textbox:fit(context, width, height)
-    setup_layout(self, width, height)
+    setup_layout(self, width, height, context.dpi)
     local ink, logical = self._layout:get_pixel_extents()
 
     if logical.width == 0 or logical.height == 0 then
@@ -161,9 +166,9 @@ local function new(text, ignore_markup)
         end
     end
 
-    local ctx = PangoCairo.font_map_get_default():create_context()
-    ctx:set_resolution(beautiful.xresources.get_dpi())
-    ret._layout = Pango.Layout.new(ctx)
+    ret._dpi = -1
+    ret._ctx = PangoCairo.font_map_get_default():create_context()
+    ret._layout = Pango.Layout.new(ret._ctx)
 
     ret:set_ellipsize("end")
     ret:set_wrap("word_char")