Rebuild Displays around the canvas, and let the transaction keep color

Claude-Session: https://claude.ai/code/session_01Ms2FbjQy31TVf3CEvQhGM8

config/dot/hypr/monitors.lua

@@ -13,8 +13,9 @@
 -- than an HDR desktop, the desktop runs SDR at 10-bit and HDR is handed to
 -- fullscreen games only, via render.cm_auto_hdr in looks.lua.
 --
--- To try full-time HDR anyway, set cm = "hdr" below (or override it in
--- overrides.lua) and read the notes in that file first.
+-- To try full-time HDR anyway, set shipped_cm = "hdr" below (or pick HDR for
+-- this display in Settings, which saves it as the display's colorProfile and
+-- wins over the shipped value) and read the notes in overrides.lua first.
 -- ─────────────────────────────────────────────────────────────────────────────
 
 local prefs = require("prefs")
@@ -23,12 +24,21 @@ local prefs = require("prefs")
 --   { ["DP-2"] = {
 --       mode = "3840x2160@60", scale = 2, transform = 0,
 --       x = 0, y = 0, primary = true,
+--       vrrMode = -1, colorProfile = "auto", bitdepth = 10,
+--       sdrBrightness = 1, sdrSaturation = 1, mirrorOf = "",
 --   } }
 --
--- Only mode, scale, and transform are read. Color management and bit depth
--- stay here, because those are the settings with a documented reason attached
--- (see the header) rather than preferences, and a settings page has no way to
--- explain the screencopy tradeoff at the moment you would be changing it.
+-- The fields past `primary` are optional: records written before they existed
+-- carry none of them, and the shipped values below stand instead. That is the
+-- relationship in both directions -- what is written here is what a saved
+-- record inherits, and what Settings saves is what overrides it, so the two
+-- stop fighting over the same monitor rule.
+--
+-- Geometry and colour fail differently on purpose. A half-written position is
+-- refused outright (below), because guessing one can strand an output where
+-- nothing can reach it. An unreadable colour, VRR or mirror value is dropped
+-- on its own and the shipped default stands: the worst it costs is a wrong
+-- shade, and taking the whole record down with it would cost the arrangement.
 local displays = prefs.get("displays", {})
 if type(displays) ~= "table" then
     displays = {}
@@ -125,19 +135,102 @@ local function display_position(entry, fallback)
     return fallback
 end
 
+local color_profiles = { auto = true, srgb = true, wide = true, hdr = true }
+
+local function color_profile(entry, fallback)
+    local value = entry ~= nil and entry.colorProfile or nil
+    if type(value) == "string" and color_profiles[value] then
+        return value
+    end
+    return fallback
+end
+
+local function bitdepth_value(entry, fallback)
+    local value = entry ~= nil and entry.bitdepth or nil
+    if value == 8 or value == 10 then
+        return value
+    end
+    return fallback
+end
+
+-- -1 means the display follows the global misc.vrr policy, which is said by
+-- leaving the key out. 3 is the global policy's own value and not a per-display
+-- choice, so it is not accepted here either.
+local function vrr_value(entry)
+    local value = entry ~= nil and entry.vrrMode or nil
+    if type(value) ~= "number" or value ~= math.floor(value)
+        or value < 0 or value > 2 then
+        return nil
+    end
+    return value
+end
+
+-- Neutral is 1.0, and the neutral value is left out rather than written: a rule
+-- that names it pins the display to it, which is not the same as leaving the
+-- trim alone.
+local function sdr_value(value, minimum, maximum)
+    if type(value) ~= "number" or value ~= value
+        or value < minimum or value > maximum
+        or math.abs(value - 1) < 0.001 then
+        return nil
+    end
+    return value
+end
+
+-- A mirror needs a target that is not itself and not another mirror -- Hyprland
+-- has no chain to follow -- and the primary may not mirror at all, since the
+-- arrangement is anchored on it.
+local function mirror_value(entry, output)
+    local value = entry ~= nil and entry.mirrorOf or nil
+    if type(value) ~= "string" or value == "" or value == output
+        or value:match("^[%w_.-]+$") == nil
+        or entry.primary == true then
+        return nil
+    end
+    local target = displays[value]
+    if type(target) == "table" and type(target.mirrorOf) == "string"
+        and target.mirrorOf ~= "" then
+        return nil
+    end
+    return value
+end
+
+-- Colour, VRR and mirroring layered onto a rule that already carries
+-- mode/position/scale/transform. A monitor rule replaces the previous rule for
+-- that output whole, so the shipped defaults are passed in here rather than
+-- written in a rule of their own. `connector` is the output name the record was
+-- saved under, which is not always the rule's own output: the Kuycon rule
+-- matches by description.
+local function with_display_fields(rule, entry, connector, default_bitdepth, default_cm)
+    rule.bitdepth = bitdepth_value(entry, default_bitdepth)
+    rule.cm = color_profile(entry, default_cm)
+    rule.vrr = vrr_value(entry)
+    rule.sdrbrightness = entry ~= nil and sdr_value(entry.sdrBrightness, 0.8, 2.0) or nil
+    rule.sdrsaturation = entry ~= nil and sdr_value(entry.sdrSaturation, 0.8, 1.2) or nil
+
+    -- A mirror shows its target's picture in its target's place, so the saved
+    -- position is not the compositor's to honour or ours to ask for.
+    local mirror = mirror_value(entry, connector)
+    if mirror ~= nil then
+        rule.mirror = mirror
+        rule.position = "auto"
+    end
+    return rule
+end
+
 -- Every connected output uses the same validated per-output store. Automatic
 -- placement and the compositor's normal color policy unless the entry says
 -- otherwise.
 for output, _ in pairs(displays) do
     local entry = display_entry(output)
     if entry ~= nil then
-        hl.monitor({
+        hl.monitor(with_display_fields({
             output = output,
             mode = entry.mode,
             position = display_position(entry, "auto"),
             scale = entry.scale,
             transform = entry.transform,
-        })
+        }, entry, output, nil, nil))
     end
 end
 
@@ -150,22 +243,23 @@ end
 local shipped_mode = "4500x3000@60"
 local shipped_scale = 1.5
 local shipped_transform = 0
+
+-- 10-bit output. 4500x3000@60 at 10bpc is ~24 Gbps, right at the edge of DP 1.4
+-- HBR3, so this relies on DSC. If the display fails to light up or falls back to
+-- a lower mode, drop this to 8 first.
+local shipped_bitdepth = 10
+
+-- "auto" = sRGB at 8bpc, wide gamut at 10bpc. Not HDR; see header.
+local shipped_cm = "auto"
+
 local kuycon = display_entry("DP-2")
-hl.monitor({
+hl.monitor(with_display_fields({
     output   = "desc:GVT Kuycon P20",
     mode     = kuycon and kuycon.mode or shipped_mode,
     position = display_position(kuycon, "0x0"),
     scale    = kuycon and kuycon.scale or shipped_scale,
     transform = kuycon and kuycon.transform or shipped_transform,
-
-    -- 10-bit output. 4500x3000@60 at 10bpc is ~24 Gbps, right at the edge of
-    -- DP 1.4 HBR3, so this relies on DSC. If the display fails to light up or
-    -- falls back to a lower mode, drop this line first.
-    bitdepth = 10,
-
-    -- "auto" = sRGB at 8bpc, wide gamut at 10bpc. Not HDR; see header.
-    cm = "auto",
-})
+}, kuycon, "DP-2", shipped_bitdepth, shipped_cm))
 
 -- Any monitor not named above: sane defaults rather than nothing.
 hl.monitor({