From 6c42aced3d6bb236a445a33b62d4331974e676dd Mon Sep 17 00:00:00 2001
From: Consolatis <35009135+Consolatis@users.noreply.github.com>
Date: Mon, 23 Jan 2023 07:15:55 +0100
Subject: [PATCH] Support flicker free boot

Fixes #724
---
 docs/labwc-config.5.scd |  7 +++++++
 docs/rc.xml.all         |  1 +
 include/config/rcxml.h  |  1 +
 src/config/rcxml.c      |  2 ++
 src/output.c            | 22 +++++++++++++++++-----
 5 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/docs/labwc-config.5.scd b/docs/labwc-config.5.scd
index 070e4c04..8407cc7c 100644
--- a/docs/labwc-config.5.scd
+++ b/docs/labwc-config.5.scd
@@ -70,6 +70,13 @@ The rest of this man page describes configuration options.
 *<core><adaptiveSync>* [yes|no]
 	Enable adaptive sync. Default is no.
 
+*<core><reuseOutputMode>* [yes|no]
+	Try to re-use the existing output mode (resolution / refresh rate).
+	This may prevent unnecessary screenblank delays when starting labwc
+	(also known as flicker free boot). If the existing output mode can not
+	be used with labwc the preferred mode of the monitor is used instead.
+	Default is no.
+
 *<core><cycleViewPreview>* [yes|no]
 	Preview the contents of the selected window when cycling between windows.
 	Default is no.
diff --git a/docs/rc.xml.all b/docs/rc.xml.all
index 2c86fb7d..d829f2a1 100644
--- a/docs/rc.xml.all
+++ b/docs/rc.xml.all
@@ -11,6 +11,7 @@
     <decoration>server</decoration>
     <gap>0</gap>
     <adaptiveSync>no</adaptiveSync>
+    <reuseOutputMode>no</reuseOutputMode>
     <cycleViewPreview>no</cycleViewPreview>
     <cycleViewOutlines>yes</cycleViewOutlines>
   </core>
diff --git a/include/config/rcxml.h b/include/config/rcxml.h
index 975e6754..56586b25 100644
--- a/include/config/rcxml.h
+++ b/include/config/rcxml.h
@@ -18,6 +18,7 @@ struct rcxml {
 	bool xdg_shell_server_side_deco;
 	int gap;
 	bool adaptive_sync;
+	bool reuse_output_mode;
 
 	/* focus */
 	bool focus_follow_mouse;
diff --git a/src/config/rcxml.c b/src/config/rcxml.c
index 068e6d3c..4d3bf6d9 100644
--- a/src/config/rcxml.c
+++ b/src/config/rcxml.c
@@ -400,6 +400,8 @@ entry(xmlNode *node, char *nodename, char *content)
 		rc.gap = atoi(content);
 	} else if (!strcasecmp(nodename, "adaptiveSync.core")) {
 		rc.adaptive_sync = get_bool(content);
+	} else if (!strcasecmp(nodename, "reuseOutputMode.core")) {
+		rc.reuse_output_mode = get_bool(content);
 	} else if (!strcmp(nodename, "name.theme")) {
 		rc.theme_name = xstrdup(content);
 	} else if (!strcmp(nodename, "cornerradius.theme")) {
diff --git a/src/output.c b/src/output.c
index 991a1653..0ae6f684 100644
--- a/src/output.c
+++ b/src/output.c
@@ -66,6 +66,12 @@ output_destroy_notify(struct wl_listener *listener, void *data)
 
 static void do_output_layout_change(struct server *server);
 
+static bool
+can_reuse_mode(struct wlr_output *wlr_output)
+{
+	return wlr_output->current_mode && wlr_output_test(wlr_output);
+}
+
 static void
 new_output_notify(struct wl_listener *listener, void *data)
 {
@@ -109,11 +115,17 @@ new_output_notify(struct wl_listener *listener, void *data)
 	wlr_log(WLR_DEBUG, "enable output");
 	wlr_output_enable(wlr_output, true);
 
-	/* The mode is a tuple of (width, height, refresh rate). */
-	wlr_log(WLR_DEBUG, "set preferred mode");
-	struct wlr_output_mode *preferred_mode =
-		wlr_output_preferred_mode(wlr_output);
-	wlr_output_set_mode(wlr_output, preferred_mode);
+	/*
+	 * Try to re-use the existing mode if configured to do so.
+	 * Failing that, try to set the preferred mode.
+	 */
+	struct wlr_output_mode *preferred_mode = NULL;
+	if (!rc.reuse_output_mode || !can_reuse_mode(wlr_output)) {
+		wlr_log(WLR_DEBUG, "set preferred mode");
+		/* The mode is a tuple of (width, height, refresh rate). */
+		preferred_mode = wlr_output_preferred_mode(wlr_output);
+		wlr_output_set_mode(wlr_output, preferred_mode);
+	}
 
 	/*
 	 * Sometimes the preferred mode is not available due to hardware