Regenerate story screen message panel blur when background changes.

src/gui/core/canvas.cpp

@@ -550,6 +550,11 @@ bool canvas::update_blur(const rect& screen_region, bool force)
 	return true;
 }
 
+void canvas::queue_reblur()
+{
+	blur_texture_.reset();
+}
+
 void canvas::draw()
 {
 	// This early-return has to come before the `validate(rect.w <= w_)` check, as during the boost_unit_tests execution

src/gui/core/canvas.hpp

@@ -101,6 +101,9 @@ public:
 	 */
 	bool update_blur(const rect& screen_region, const bool force = false);
 
+	/** Clear the cached blur texture, forcing it to regenerate. */
+	void queue_reblur();
+
 	/**
 	 * Draw the canvas' shapes onto the screen.
 	 *

src/gui/dialogs/story_viewer.cpp

@@ -307,6 +307,9 @@ void story_viewer::display_part()
 	text_label.set_text_alpha(0);
 	text_label.set_label(part_text);
 
+	// Regenerate any background blur texture
+	panel_canvas.queue_reblur();
+
 	begin_fade_draw(true);
 	// if the previous page was skipped, it is possible that we already have a timer running.
 	clear_image_timer();