Add support for restoring the view position in reloadable views

The status view still uses its own more specialized restoring code.
There are still some cases which are not completely handled. For
example, restoring will only be done when the current line is the first
in the view, thus changing the view position will cancel the restoring.
However, if you change back to the first line restoring will be enabled.

NEWS

@@ -13,6 +13,7 @@ Improvements:
 
  - Tree view: avoid flickering when updating.
  - Tree & blob view: open any blob in an editor.
+ - Stage & main view: restore view position when reloading.
 
 Bug fixes:
 

tig.c

@@ -1725,6 +1725,9 @@ struct view {
 	/* Navigation */
 	unsigned long offset;	/* Offset of the window top */
 	unsigned long lineno;	/* Current line number */
+	unsigned long p_offset;	/* Previous offset of the window top */
+	unsigned long p_lineno;	/* Previous current line number */
+	bool p_restore;		/* Should the previous position be restored. */
 
 	/* Searching */
 	char grep[SIZEOF_STR];	/* Search string */
@@ -2516,6 +2519,9 @@ reset_view(struct view *view)
 		free(view->line[i].data);
 	free(view->line);
 
+	view->p_offset = view->offset;
+	view->p_lineno = view->lineno;
+
 	view->line = NULL;
 	view->offset = 0;
 	view->lines  = 0;
@@ -2597,6 +2603,43 @@ format_argv(const char *dst_argv[], const char *src_argv[], enum format_flags fl
 	return src_argv[argc] == NULL;
 }
 
+static bool
+restore_view_position(struct view *view)
+{
+	if (!view->p_restore || (view->pipe && view->lines <= view->p_lineno))
+		return FALSE;
+
+	/* Changing the view position cancels the restoring. */
+	/* FIXME: Changing back to the first line is not detected. */
+	if (view->offset != 0 || view->lineno != 0) {
+		view->p_restore = FALSE;
+		return FALSE;
+	}
+
+	if (view->p_lineno >= view->lines) {
+		view->p_lineno = view->lines > 0 ? view->lines - 1 : 0;
+		if (view->p_offset >= view->p_lineno) {
+			unsigned long half = view->height / 2;
+
+			if (view->p_lineno > half)
+				view->p_offset = view->p_lineno - half;
+			else
+				view->p_offset = 0;
+		}
+	}
+
+	if (view_is_displayed(view) &&
+	    view->offset != view->p_offset &&
+	    view->lineno != view->p_lineno)
+		werase(view->win);
+
+	view->offset = view->p_offset;
+	view->lineno = view->p_lineno;
+	view->p_restore = FALSE;
+
+	return TRUE;
+}
+
 static void
 end_update(struct view *view, bool force)
 {
@@ -2768,6 +2811,9 @@ update_view(struct view *view)
 		end_update(view, FALSE);
 	}
 
+	if (restore_view_position(view))
+		redraw = TRUE;
+
 	if (!view_is_displayed(view))
 		return TRUE;
 
@@ -2883,6 +2929,7 @@ open_view(struct view *prev, enum request request, enum open_flags flags)
 			report("Failed to load %s view", view->name);
 			return;
 		}
+		restore_view_position(view);
 
 	} else if ((reload || strcmp(view->vid, view->id)) &&
 		   !begin_update(view, flags & (OPEN_REFRESH | OPEN_PREPARED))) {
@@ -2912,6 +2959,7 @@ open_view(struct view *prev, enum request request, enum open_flags flags)
 		/* Clear the old view and let the incremental updating refill
 		 * the screen. */
 		werase(view->win);
+		view->p_restore = flags & (OPEN_RELOAD | OPEN_REFRESH);
 		report("");
 	} else if (view_is_displayed(view)) {
 		redraw_view(view);
@@ -4358,14 +4406,38 @@ static const char *update_index_argv[] = {
 	"git", "update-index", "-q", "--unmerged", "--refresh", NULL
 };
 
+/* Restore the previous line number to stay in the context or select a
+ * line with something that can be updated. */
+static void
+status_restore(struct view *view)
+{
+	if (view->p_lineno >= view->lines)
+		view->p_lineno = view->lines - 1;
+	while (view->p_lineno < view->lines && !view->line[view->p_lineno].data)
+		view->p_lineno++;
+	while (view->p_lineno > 0 && !view->line[view->p_lineno].data)
+		view->p_lineno--;
+
+	/* If the above fails, always skip the "On branch" line. */
+	if (view->p_lineno < view->lines)
+		view->lineno = view->p_lineno;
+	else
+		view->lineno = 1;
+
+	if (view->lineno < view->offset)
+		view->offset = view->lineno;
+	else if (view->offset + view->height <= view->lineno)
+		view->offset = view->lineno - view->height + 1;
+
+	view->p_restore = FALSE;
+}
+
 /* First parse staged info using git-diff-index(1), then parse unstaged
  * info using git-diff-files(1), and finally untracked files using
  * git-ls-files(1). */
 static bool
 status_open(struct view *view)
 {
-	unsigned long prev_lineno = view->lineno;
-
 	reset_view(view);
 
 	add_line_data(view, NULL, LINE_STAT_HEAD);
@@ -4389,27 +4461,10 @@ status_open(struct view *view)
 	    !status_run(view, status_list_other_argv, '?', LINE_STAT_UNTRACKED))
 		return FALSE;
 
-	/* If all went well restore the previous line number to stay in
-	 * the context or select a line with something that can be
-	 * updated. */
-	if (prev_lineno >= view->lines)
-		prev_lineno = view->lines - 1;
-	while (prev_lineno < view->lines && !view->line[prev_lineno].data)
-		prev_lineno++;
-	while (prev_lineno > 0 && !view->line[prev_lineno].data)
-		prev_lineno--;
-
-	/* If the above fails, always skip the "On branch" line. */
-	if (prev_lineno < view->lines)
-		view->lineno = prev_lineno;
-	else
-		view->lineno = 1;
-
-	if (view->lineno < view->offset)
-		view->offset = view->lineno;
-	else if (view->offset + view->height <= view->lineno)
-		view->offset = view->lineno - view->height + 1;
-
+	/* Restore the exact position or use the specialized restore
+	 * mode? */
+	if (!view->p_restore)
+		status_restore(view);
 	return TRUE;
 }
 
@@ -4548,7 +4603,7 @@ status_enter(struct view *view, struct line *line)
 	}
 
 	split = view_is_displayed(view) ? OPEN_SPLIT : 0;
-	open_view(view, REQ_VIEW_STAGE, OPEN_REFRESH | split);
+	open_view(view, REQ_VIEW_STAGE, OPEN_PREPARED | split);
 	if (view_is_displayed(VIEW(REQ_VIEW_STAGE))) {
 		if (status) {
 			stage_status = *status;
@@ -5101,12 +5156,15 @@ stage_request(struct view *view, enum request request, struct line *line)
 		return request;
 	}
 
+	VIEW(REQ_VIEW_STATUS)->p_restore = TRUE;
 	open_view(view, REQ_VIEW_STATUS, OPEN_RELOAD | OPEN_NOMAXIMIZE);
 
 	/* Check whether the staged entry still exists, and close the
 	 * stage view if it doesn't. */
-	if (!status_exists(&stage_status, stage_line_type))
+	if (!status_exists(&stage_status, stage_line_type)) {
+		status_restore(VIEW(REQ_VIEW_STATUS));
 		return REQ_VIEW_CLOSE;
+	}
 
 	if (stage_line_type == LINE_STAT_UNTRACKED) {
 	    	if (!suffixcmp(stage_status.new.name, -1, "/")) {