UI: Clamp scrollbar offset to lower view boundaries

Fixes the "emtpy scrolling" glitch by clamping the scroller offset to
the boundary of the view when it's smaller than the previous.

Fixes T45197. Patch by @januz.

Differential Revision: D1580

1 files changed, 19 insertions, 5 deletions

diff --git a/source/blender/editors/screen/area.c b/source/blender/editors/screen/area.c
index caae803100b..a53a16906d3 100644
--- a/source/blender/editors/screen/area.c
+++ b/source/blender/editors/screen/area.c
@@ -1939,15 +1939,29 @@ void ED_region_panels(const bContext *C, ARegion *ar, const char *context, int c
 		/* before setting the view */
 		if (vertical) {
 			/* we always keep the scroll offset - so the total view gets increased with the scrolled away part */
-			if (v2d->cur.ymax < - 0.001f)
-				y = min_ii(y, v2d->cur.ymin);
-			
+			if (v2d->cur.ymax < -FLT_EPSILON) {
+				/* Clamp to lower view boundary */
+				if (v2d->tot.ymin < -v2d->winy) {
+					y = min_ii(y, 0);
+				}
+				else {
+					y = min_ii(y, v2d->cur.ymin);
+				}
+			}
+
 			y = -y;
 		}
 		else {
 			/* don't jump back when panels close or hide */
-			if (!is_context_new)
-				x = max_ii(x, v2d->cur.xmax);
+			if (!is_context_new) {
+				if (v2d->tot.xmax > v2d->winx) {
+					x = max_ii(x, 0);
+				}
+				else {
+					x = max_ii(x, v2d->cur.xmax);
+				}
+			}
+
 			y = -y;
 		}