screen: Disable trying to auto-compute the DPI by default.

As long as Awesome provides APIs that uses pixels are points, this
cannot be enabled by default.

For example, a wibar size defined in pixels may be too small to
render the text once a dense display is connected.

lib/awful/screen.lua

@@ -504,25 +504,78 @@ function screen.object.get_selected_tag(s)
     return screen.object.get_selected_tags(s)[1]
 end
 
+--- Enable the automatic calculation of the screen DPI (experimental).
+--
+-- This will cause many elements such as the font and some widgets to be scaled
+-- so they look the same (physical) size on different devices with different
+-- pixel density.
+--
+-- It is calculated using the information provided from `xrandr`.
+--
+-- When enabled, the theme and configuration must avoid using pixel sizes for
+-- different elements as this will cause misalignment or hidden content on some
+-- devices.
+--
+-- Note that it has to be called early in `rc.lua` and requires restarting
+-- awesome to take effect. It is disabled by default and changes introduced in
+-- minor releases of Awesome may slightly break the behavior as more components
+-- gain support for HiDPI.
+--
+-- When disabled the DPI is acquired from the `Xft.dpi` X resource (xrdb),
+-- defaulting to 96.
+--
+-- @tparam boolean enabled Enable or disable automatic DPI support.
+function screen.set_auto_dpi_enabled(enabled)
+    for s in capi.screen do
+        s.data.dpi_cache = nil
+    end
+    data.autodpi = enabled
+end
+
+
 --- The number of pixels per inch of the screen.
 -- @property dpi
 -- @treturn number the DPI value.
 
 local xft_dpi, fallback_dpi
 
+local function get_fallback()
+    local mm_per_inch = 25.4
+
+    -- Following Keith Packard's whitepaper on Xft,
+    -- https://keithp.com/~keithp/talks/xtc2001/paper/xft.html#sec-editing
+    -- the proper fallback for Xft.dpi is the vertical DPI reported by
+    -- the X server. This will generally be 96 on Xorg, unless the user
+    -- has configured it differently
+    if root and not fallback_dpi then
+        local _, h = root.size()
+        local _, hmm = root.size_mm()
+        fallback_dpi = hmm ~= 0 and h * mm_per_inch / hmm
+    end
+
+    return fallback_dpi or 96
+end
+
 function screen.object.get_dpi(s)
     local mm_per_inch = 25.4
 
-    if s.data.dpi then
+    if s.data.dpi or s.data.dpi_cache then
-        return s.data.dpi
+        return s.data.dpi or s.data.dpi_cache
     end
 
     -- Xft.dpi is explicit user configuration, so honor it
     if not xft_dpi and awesome and awesome.xrdb_get_value then
         xft_dpi = tonumber(awesome.xrdb_get_value("", "Xft.dpi")) or false
     end
+
     if xft_dpi then
-        return xft_dpi
+        s.data.dpi_cache = xft_dpi
+        return s.data.dpi_cache
+    end
+
+    if not data.autodpi then
+        s.data.dpi_cache = get_fallback()
+        return s.data.dpi_cache
     end
 
     -- Try to compute DPI based on outputs (use the minimum)
@@ -537,16 +590,12 @@ function screen.object.get_dpi(s)
         end
     end
     if dpi then
+        s.data.dpi_cache = dpi
         return dpi
     end
 
-    -- We have no outputs, so guess based on the size of the root window.
+    s.data.dpi_cache = get_fallback()
-    if root and not fallback_dpi then
+    return s.data.dpi_cache
-        local _, h = root.size()
-        local _, hmm = root.size_mm()
-        fallback_dpi = hmm ~= 0 and h * mm_per_inch / hmm
-    end
-    return fallback_dpi or 96
 end
 
 function screen.object.set_dpi(s, dpi)

lib/beautiful/xresources.lua

@@ -69,8 +69,14 @@ local function get_screen(s)
 end
 
 --- Get global or per-screen DPI value falling back to xrdb.
+--
+-- This function is deprecated. Use `s.dpi` and avoid getting the DPI without
+-- a screen.
+--
+-- @deprecated xresources.get_dpi
 -- @tparam[opt] integer|screen s The screen.
 -- @treturn number DPI value.
+
 function xresources.get_dpi(s)
     s = get_screen(s)
     if s then