Fix T42495: Fullscreen area icon glitch

Tried a couple of things to trigger an update/redraw for the exact right
moment (sending azone update event, timer, delayed redraw, etc) but this
seems to work rock solid without being *that* ugly.

source/blender/editors/include/ED_screen.h

@@ -86,6 +86,7 @@ void    ED_area_tag_redraw(ScrArea *sa);
 void    ED_area_tag_redraw_regiontype(ScrArea *sa, int type);
 void    ED_area_tag_refresh(ScrArea *sa);
 void    ED_area_do_refresh(struct bContext *C, ScrArea *sa);
+void    ED_area_azones_update(ScrArea *sa, const int mouse_xy[]);
 void    ED_area_headerprint(ScrArea *sa, const char *str);
 void    ED_area_newspace(struct bContext *C, ScrArea *sa, int type);
 void    ED_area_prevspace(struct bContext *C, ScrArea *sa);

source/blender/editors/screen/area.c

@@ -151,6 +151,34 @@ void ED_area_do_refresh(bContext *C, ScrArea *sa)
 	sa->do_refresh = false;
 }
 
+/**
+ * Action zones are only updated if the mouse is inside of them, but in some cases (currently only fullscreen icon)
+ * it might be needed to update their properties and redraw if the mouse isn't inside.
+ */
+void ED_area_azones_update(ScrArea *sa, const int mouse_xy[2])
+{
+	AZone *az;
+	bool changed = false;
+
+	for (az = sa->actionzones.first; az; az = az->next) {
+		if (az->type == AZONE_FULLSCREEN) {
+			/* only if mouse is not hovering the azone */
+			if (BLI_rcti_isect_pt_v(&az->rect, mouse_xy) == false) {
+				az->alpha = 0.0f;
+				changed = true;
+
+				/* can break since currently only this is handled here */
+				break;
+			}
+		}
+	}
+
+	if (changed) {
+		sa->flag &= ~AREA_FLAG_ACTIONZONES_UPDATE;
+		ED_area_tag_redraw(sa);
+	}
+}
+
 /**
  * \brief Corner widget use for quitting fullscreen.
  */
@@ -369,6 +397,11 @@ static void region_draw_azone_tria(AZone *az)
 	glDisable(GL_BLEND);
 }
 
+static void area_azone_tag_update(ScrArea *sa)
+{
+	sa->flag |= AREA_FLAG_ACTIONZONES_UPDATE;
+}
+
 static void region_draw_azones(ScrArea *sa, ARegion *ar)
 {
 	AZone *az;
@@ -409,6 +442,10 @@ static void region_draw_azones(ScrArea *sa, ARegion *ar)
 			}
 			else if (az->type == AZONE_FULLSCREEN) {
 				area_draw_azone_fullscreen(az->x1, az->y1, az->x2, az->y2, az->alpha);
+
+				if (az->alpha != 0.0f) {
+					area_azone_tag_update(sa);
+				}
 			}
 		}
 	}

source/blender/makesdna/DNA_screen_types.h

@@ -281,6 +281,8 @@ enum {
 	AREA_FLAG_TEMP_TYPE          = (1 << 6),
 	/* for temporary fullscreens (file browser, image editor render) that are opened above user set fullscreens */
 	AREA_FLAG_STACKED_FULLSCREEN = (1 << 7),
+	/* update action zones (even if the mouse is not intersecting them) */
+	AREA_FLAG_ACTIONZONES_UPDATE = (1 << 8),
 };
 
 #define EDGEWIDTH	1

source/blender/windowmanager/intern/wm_event_system.c

@@ -2330,6 +2330,11 @@ void wm_event_do_handlers(bContext *C)
 						break;
 					}
 
+					/* update azones if needed - done here because it needs to be independent from redraws */
+					if (sa->flag & AREA_FLAG_ACTIONZONES_UPDATE) {
+						ED_area_azones_update(sa, &event->x);
+					}
+
 					if (wm_event_inside_i(event, &sa->totrct)) {
 						CTX_wm_area_set(C, sa);