From 2627ab322e0e8dffbf86b1c2eb969139a8062174 Mon Sep 17 00:00:00 2001
From: nicm <nicm>
Date: Fri, 2 Sep 2016 20:57:20 +0000
Subject: [PATCH] Remember the number of lines scrolled into the history
 (versus cleared into the history) and when resizing only use scrolled lines
 and not cleared lines (which are probably not intended to reappear). From
 Chaoren Lin.

---
 grid-view.c    |  1 +
 grid.c         | 11 +++++++++++
 screen-write.c |  2 +-
 screen.c       | 15 +++++++++------
 tmux.h         |  1 +
 5 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/grid-view.c b/grid-view.c
index 0989f800..8160679d 100644
--- a/grid-view.c
+++ b/grid-view.c
@@ -67,6 +67,7 @@ grid_view_clear_history(struct grid *gd)
 		grid_collect_history(gd);
 		grid_scroll_history(gd);
 	}
+	gd->hscrolled = 0;
 }
 
 /* Clear area. */
diff --git a/grid.c b/grid.c
index f0eda7e0..50766a88 100644
--- a/grid.c
+++ b/grid.c
@@ -99,6 +99,7 @@ grid_create(u_int sx, u_int sy, u_int hlimit)
 
 	gd->flags = GRID_HISTORY;
 
+	gd->hscrolled = 0;
 	gd->hsize = 0;
 	gd->hlimit = hlimit;
 
@@ -170,6 +171,8 @@ grid_collect_history(struct grid *gd)
 
 	grid_move_lines(gd, 0, yy, gd->hsize + gd->sy - yy);
 	gd->hsize -= yy;
+	if (gd->hscrolled > gd->hsize)
+		gd->hscrolled = gd->hsize;
 }
 
 /*
@@ -186,6 +189,7 @@ grid_scroll_history(struct grid *gd)
 	    sizeof *gd->linedata);
 	memset(&gd->linedata[yy], 0, sizeof gd->linedata[yy]);
 
+	gd->hscrolled++;
 	gd->hsize++;
 }
 
@@ -196,7 +200,9 @@ grid_clear_history(struct grid *gd)
 	grid_clear_lines(gd, 0, gd->hsize);
 	grid_move_lines(gd, 0, gd->hsize, gd->sy);
 
+	gd->hscrolled = 0;
 	gd->hsize = 0;
+
 	gd->linedata = xreallocarray(gd->linedata, gd->sy,
 	    sizeof *gd->linedata);
 }
@@ -231,6 +237,7 @@ grid_scroll_history_region(struct grid *gd, u_int upper, u_int lower)
 	memset(gl_lower, 0, sizeof *gl_lower);
 
 	/* Move the history offset down over the line. */
+	gd->hscrolled++;
 	gd->hsize++;
 }
 
@@ -914,6 +921,10 @@ grid_reflow(struct grid *dst, struct grid *src, u_int new_x)
 			grid_reflow_join(dst, &py, src_gl, new_x);
 		}
 		previous_wrapped = (src_gl->flags & GRID_LINE_WRAPPED);
+
+		/* This is where we started scrolling. */
+		if (line == sy + src->hsize - src->hscrolled - 1)
+			dst->hscrolled = 0;
 	}
 
 	grid_destroy(src);
diff --git a/screen-write.c b/screen-write.c
index 3c4d2758..94ce359f 100644
--- a/screen-write.c
+++ b/screen-write.c
@@ -1016,7 +1016,7 @@ screen_write_clearhistory(struct screen_write_ctx *ctx)
 	struct grid	*gd = s->grid;
 
 	grid_move_lines(gd, 0, gd->hsize, gd->sy);
-	gd->hsize = 0;
+	gd->hscrolled = gd->hsize = 0;
 }
 
 /* Write cell data. */
diff --git a/screen.c b/screen.c
index f5f39d37..8e4f8a65 100644
--- a/screen.c
+++ b/screen.c
@@ -177,8 +177,9 @@ screen_resize_y(struct screen *s, u_int sy)
 	 * If the height is decreasing, delete lines from the bottom until
 	 * hitting the cursor, then push lines from the top into the history.
 	 *
-	 * When increasing, pull as many lines as possible from the history to
-	 * the top, then fill the remaining with blanks at the bottom.
+	 * When increasing, pull as many lines as possible from scrolled
+	 * history (not explicitly cleared from view) to the top, then fill the
+	 * remaining with blanks at the bottom.
 	 */
 
 	/* Size decreasing. */
@@ -200,9 +201,10 @@ screen_resize_y(struct screen *s, u_int sy)
 		 * lines from the top.
 		 */
 		available = s->cy;
-		if (gd->flags & GRID_HISTORY)
+		if (gd->flags & GRID_HISTORY) {
+			gd->hscrolled += needed;
 			gd->hsize += needed;
-		else if (needed > 0 && available > 0) {
+		} else if (needed > 0 && available > 0) {
 			if (available > needed)
 				available = needed;
 			grid_view_delete_lines(gd, 0, available);
@@ -219,13 +221,14 @@ screen_resize_y(struct screen *s, u_int sy)
 		needed = sy - oldy;
 
 		/*
-		 * Try to pull as much as possible out of the history, if is
+		 * Try to pull as much as possible out of scrolled history, if is
 		 * is enabled.
 		 */
-		available = gd->hsize;
+		available = gd->hscrolled;
 		if (gd->flags & GRID_HISTORY && available > 0) {
 			if (available > needed)
 				available = needed;
+			gd->hscrolled -= available;
 			gd->hsize -= available;
 			s->cy += available;
 		} else
diff --git a/tmux.h b/tmux.h
index f9ce0309..d1879e54 100644
--- a/tmux.h
+++ b/tmux.h
@@ -691,6 +691,7 @@ struct grid {
 	u_int			 sx;
 	u_int			 sy;
 
+	u_int			 hscrolled;
 	u_int			 hsize;
 	u_int			 hlimit;