snap: cache and ignore last-snapped edge when growing or shrinking

When growing or shrinking a view by snapping to an edge, a client may
ignore the requested size and instead keep its original size or
substitute a different (possibly constrained) size. In this case, the
view may not actually contact the snapped edge, and a subsequent snap
attempt will just keep re-trying (and failing) to contact the same ege.

To mitigate this, remember the last-snapped view, snapping direction and
offset of the snapping edge in snap.c; when re-attempting a snap for the
same view in the same direction, ignore the edge that was last "hit", to
allow snapping to progress beyond the problematic edge.

include/edges.h

@@ -105,12 +105,12 @@ void edges_adjust_geom(struct view *view, struct border edges,
 	uint32_t resize_edges, struct wlr_box *geom);
 
 void edges_find_neighbors(struct border *nearest_edges, struct view *view,
-	struct wlr_box target, struct output *output,
-	edge_validator_t validator, bool use_pending, bool ignore_hidden);
+	struct wlr_box origin, struct wlr_box target,
+	struct output *output, edge_validator_t validator, bool ignore_hidden);
 
 void edges_find_outputs(struct border *nearest_edges, struct view *view,
-	struct wlr_box target, struct output *output,
-	edge_validator_t validator, bool use_pending);
+	struct wlr_box origin, struct wlr_box target,
+	struct output *output, edge_validator_t validator);
 
 void edges_adjust_move_coords(struct view *view, struct border edges,
 	int *x, int *y, bool use_pending);

include/snap-constraints.h

@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_SNAP_CONSTRAINTS_H
+#define LABWC_SNAP_CONSTRAINTS_H
+
+#include "common/border.h"
+#include "view.h"
+
+struct wlr_box;
+
+void snap_constraints_set(struct view *view,
+	enum view_edge direction, struct wlr_box geom);
+
+void snap_constraints_invalidate(struct view *view);
+
+void snap_constraints_update(struct view *view);
+
+struct wlr_box snap_constraints_effective(struct view *view,
+	enum view_edge direction);
+
+#endif /* LABWC_SNAP_CONSTRAINTS_H */

include/snap.h

@@ -16,4 +16,7 @@ void snap_grow_to_next_edge(struct view *view,
 void snap_shrink_to_next_edge(struct view *view,
 	enum view_edge direction, struct wlr_box *geo);
 
+void snap_invalidate_edge_cache(struct view *view);
+void snap_update_cache_geometry(struct view *view);
+
 #endif /* LABWC_SNAP_H */

src/edges.c

@@ -376,8 +376,8 @@ edges_calculate_visibility(struct server *server, struct view *ignored_view)
 
 void
 edges_find_neighbors(struct border *nearest_edges, struct view *view,
-		struct wlr_box target, struct output *output,
-		edge_validator_t validator, bool use_pending, bool ignore_hidden)
+		struct wlr_box origin, struct wlr_box target,
+		struct output *output, edge_validator_t validator, bool ignore_hidden)
 {
 	assert(view);
 	assert(validator);
@@ -391,10 +391,7 @@ edges_find_neighbors(struct border *nearest_edges, struct view *view,
 	struct border view_edges = { 0 };
 	struct border target_edges = { 0 };
 
-	struct wlr_box *view_geom =
-		use_pending ? &view->pending : &view->current;
-
-	edges_for_target_geometry(&view_edges, view, *view_geom);
+	edges_for_target_geometry(&view_edges, view, origin);
 	edges_for_target_geometry(&target_edges, view, target);
 
 	struct view *v;
@@ -437,8 +434,8 @@ edges_find_neighbors(struct border *nearest_edges, struct view *view,
 
 void
 edges_find_outputs(struct border *nearest_edges, struct view *view,
-		struct wlr_box target, struct output *output,
-		edge_validator_t validator, bool use_pending)
+		struct wlr_box origin, struct wlr_box target,
+		struct output *output, edge_validator_t validator)
 {
 	assert(view);
 	assert(validator);
@@ -453,10 +450,7 @@ edges_find_outputs(struct border *nearest_edges, struct view *view,
 	struct border view_edges = { 0 };
 	struct border target_edges = { 0 };
 
-	struct wlr_box *view_geom =
-		use_pending ? &view->pending : &view->current;
-
-	edges_for_target_geometry(&view_edges, view, *view_geom);
+	edges_for_target_geometry(&view_edges, view, origin);
 	edges_for_target_geometry(&target_edges, view, target);
 
 	struct output *o;
@@ -473,7 +467,7 @@ edges_find_outputs(struct border *nearest_edges, struct view *view,
 			output_usable_area_in_layout_coords(o);
 
 		struct wlr_box ol;
-		if (!wlr_box_intersection(&ol, view_geom, &usable) &&
+		if (!wlr_box_intersection(&ol, &origin, &usable) &&
 				!wlr_box_intersection(&ol, &target, &usable)) {
 			continue;
 		}

src/interactive.c

@@ -4,6 +4,7 @@
 #include "labwc.h"
 #include "regions.h"
 #include "resize_indicator.h"
+#include "snap.h"
 #include "view.h"
 #include "window-rules.h"
 

src/meson.build

@@ -21,6 +21,7 @@ labwc_sources = files(
   'seat.c',
   'server.c',
   'session-lock.c',
+  'snap-constraints.c',
   'snap.c',
   'tearing.c',
   'theme.c',

src/resistance.c

@@ -108,15 +108,14 @@ resistance_move_apply(struct view *view, double *x, double *y)
 
 	if (rc.screen_edge_strength != 0) {
 		/* Find any relevant output edges encountered by this move */
-		edges_find_outputs(&next_edges, view, target, NULL,
-			check_edge_output, /* use_pending */ false);
+		edges_find_outputs(&next_edges, view,
+			view->current, target, NULL, check_edge_output);
 	}
 
 	if (rc.window_edge_strength != 0) {
 		/* Find any relevant window edges encountered by this move */
-		edges_find_neighbors(&next_edges,
-			view, target, NULL, check_edge_window,
-			/* use_pending */ false, /* ignore_hidden */ true);
+		edges_find_neighbors(&next_edges, view, view->current, target,
+			NULL, check_edge_window, /* ignore_hidden */ true);
 	}
 
 	/* If any "best" edges were encountered during this move, snap motion */
@@ -138,15 +137,14 @@ resistance_resize_apply(struct view *view, struct wlr_box *new_geom)
 
 	if (rc.screen_edge_strength != 0) {
 		/* Find any relevant output edges encountered by this move */
-		edges_find_outputs(&next_edges, view, *new_geom, NULL,
-			check_edge_output, /* use_pending */ false);
+		edges_find_outputs(&next_edges, view,
+			view->current, *new_geom, NULL, check_edge_output);
 	}
 
 	if (rc.window_edge_strength != 0) {
 		/* Find any relevant window edges encountered by this move */
-		edges_find_neighbors(&next_edges,
-			view, *new_geom, NULL, check_edge_window,
-			/* use_pending */ false, /* ignore_hidden */ true);
+		edges_find_neighbors(&next_edges, view, view->current, *new_geom,
+			NULL, check_edge_window, /* ignore_hidden */ true);
 	}
 
 	/* If any "best" edges were encountered during this move, snap motion */

src/snap-constraints.c

@@ -0,0 +1,189 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <assert.h>
+#include <wlr/util/box.h>
+#include "common/macros.h"
+#include "labwc.h"
+#include "snap-constraints.h"
+#include "view.h"
+
+/*
+ * When snapping to edges during a resize action, XDG clients can override the
+ * desired size to honor arbitrary sizing constraints (*e.g.*, honoring some
+ * aspect ratio or ensuring that a terminal window consists of an integer
+ * number of character cells). In that case, the final position of the view
+ * edge may fall short of the edge to which it's snapped.
+ *
+ * This can prevent a subsequent resize action in the same direction from ever
+ * crossing the "missed" edge, because the next action will keep trying to hit
+ * the edge and the client will overriding the desired size. To compensate,
+ * every snapped resize will update a "last snapped" cache, recording the
+ * view that was resized, the position (and orientation) of the resized edge,
+ * and the expected geometry resulting from the snapped resize. At first, the
+ * expected geometry is the "pending" geometry that will be sent in a configure
+ * request. However, if the client overrides this pending geometry with some
+ * other, constrained value, the expectation should be updated (only once!) to
+ * reflect the size that the client chooses to honor.
+ *
+ * In subsequent snapped resize actions, if:
+ *
+ * 1. The view is the same as in the last attemped snapped resize;
+ * 2. The direction of resizing is the same as in the last attempt; and
+ * 3. The geometry of the view matches that expected from the last attempt;
+ *
+ * then the view geometry will be modified to reflect the *original* intended
+ * geometry from last snapped resize, which will allow the current attempt to
+ * progress beyond the "sticky" edge.
+ */
+static struct {
+	struct view *view;
+	bool pending;
+	int offset;
+	enum view_edge direction;
+	struct wlr_box geom;
+} last_snap_hit;
+
+static void
+snap_constraints_reset(void)
+{
+	last_snap_hit.view = NULL;
+	last_snap_hit.pending = false;
+	last_snap_hit.offset = INT_MIN;
+	last_snap_hit.direction = VIEW_EDGE_INVALID;
+	memset(&last_snap_hit.geom, 0, sizeof(last_snap_hit.geom));
+}
+
+static bool
+snap_constraints_are_valid(struct view *view, enum view_edge direction)
+{
+	assert(view);
+
+	/* Cache is not valid if view has changed */
+	if (view != last_snap_hit.view) {
+		return false;
+	}
+
+	/* Cache is not valid if direction has changed */
+	if (direction != last_snap_hit.direction) {
+		return false;
+	}
+
+	/* Cache is not valid if offset is unbounded */
+	if (!BOUNDED_INT(last_snap_hit.offset)) {
+		return false;
+	}
+
+	/* Cache is valid iff pending view geometry matches expectation */
+	return wlr_box_equal(&view->pending, &last_snap_hit.geom);
+}
+
+void
+snap_constraints_set(struct view *view,
+		enum view_edge direction, struct wlr_box geom)
+{
+	assert(view);
+
+	int offset = INT_MIN;
+	switch (direction) {
+	case VIEW_EDGE_LEFT:
+		offset = geom.x;
+		break;
+	case VIEW_EDGE_RIGHT:
+		offset = geom.x + geom.width;
+		break;
+	case VIEW_EDGE_UP:
+		offset = geom.y;
+		break;
+	case VIEW_EDGE_DOWN:
+		offset = geom.y + geom.height;
+		break;
+	default:
+		break;
+	}
+
+	if (!BOUNDED_INT(offset)) {
+		snap_constraints_reset();
+		return;
+	}
+
+	/* Capture the current geometry and effective snapped edge */
+	last_snap_hit.view = view;
+	last_snap_hit.offset = offset;
+	last_snap_hit.direction = direction;
+	last_snap_hit.geom = geom;
+
+	/*
+	 * Client geometry change is pending, and XDG clients may adjust the
+	 * geometry to match arbitrary constraints. Allow the client to update
+	 * our concept of constraints exactly once after the configure request.
+	 */
+	last_snap_hit.pending = true;
+}
+
+void
+snap_constraints_invalidate(struct view *view)
+{
+	if (view == last_snap_hit.view) {
+		snap_constraints_reset();
+	}
+}
+
+void
+snap_constraints_update(struct view *view)
+{
+	assert(view);
+
+	/* Ignore update if this is the wrong view */
+	if (view != last_snap_hit.view) {
+		return;
+	}
+
+	/* Never update constraints more than once */
+	if (!last_snap_hit.pending) {
+		return;
+	}
+
+	/* Only update constraints when view geometry matches expectation */
+	if (!wlr_box_equal(&view->pending, &last_snap_hit.geom)) {
+		return;
+	}
+
+	last_snap_hit.geom = view->current;
+	last_snap_hit.pending = false;
+}
+
+struct wlr_box
+snap_constraints_effective(struct view *view, enum view_edge direction)
+{
+	assert(view);
+
+	/* Use actual geometry when constraints do not apply */
+	if (!snap_constraints_are_valid(view, direction)) {
+		return view->pending;
+	}
+
+	/* Override changing edge with constrained value */
+	struct wlr_box geom = view->pending;
+	switch (last_snap_hit.direction) {
+	case VIEW_EDGE_LEFT:
+		geom.x = last_snap_hit.offset;
+		break;
+	case VIEW_EDGE_RIGHT:
+		geom.width = last_snap_hit.offset - geom.x;
+		break;
+	case VIEW_EDGE_UP:
+		geom.y = last_snap_hit.offset;
+		break;
+	case VIEW_EDGE_DOWN:
+		geom.height = last_snap_hit.offset - geom.y;
+		break;
+	default:
+		return view->pending;
+	}
+
+	/* Fall back to actual geometry when constrained geometry is nonsense */
+	if (geom.width <= 0 || geom.height <= 0) {
+		return view->pending;
+	}
+
+	return geom;
+}

src/snap.c

@@ -4,10 +4,9 @@
 #include <wlr/util/box.h>
 #include "common/border.h"
 #include "common/macros.h"
-#include "config/rcxml.h"
 #include "edges.h"
 #include "labwc.h"
-#include "resistance.h"
+#include "snap-constraints.h"
 #include "snap.h"
 #include "view.h"
 
@@ -121,9 +120,8 @@ snap_move_to_edge(struct view *view, enum view_edge direction,
 		struct border next_edges;
 		edges_initialize(&next_edges);
 
-		edges_find_neighbors(&next_edges,
-			view, target, output, check_edge,
-			/* use_pending */ true, /* ignore_hidden */ false);
+		edges_find_neighbors(&next_edges, view, view->pending, target,
+			output, check_edge, /* ignore_hidden */ false);
 
 		/* If any "best" edges were encountered, limit motion */
 		edges_adjust_move_coords(view, next_edges,
@@ -135,8 +133,8 @@ snap_move_to_edge(struct view *view, enum view_edge direction,
 }
 
 void
-snap_grow_to_next_edge(struct view *view, enum view_edge direction,
-		struct wlr_box *geo)
+snap_grow_to_next_edge(struct view *view,
+		enum view_edge direction, struct wlr_box *geo)
 {
 	assert(view);
 	assert(!view->shaded);
@@ -196,17 +194,26 @@ snap_grow_to_next_edge(struct view *view, enum view_edge direction,
 	struct border next_edges;
 	edges_initialize(&next_edges);
 
+	/* Use a constrained, effective geometry for snapping if appropriate */
+	struct wlr_box origin = snap_constraints_effective(view, direction);
+
 	/* Limit motion to any intervening edge of other views on this output */
-	edges_find_neighbors(&next_edges,
-		view, *geo, output, check_edge,
-		/* use_pending */ true, /* ignore_hidden */ false);
+	edges_find_neighbors(&next_edges, view, origin, *geo,
+		output, check_edge, /* ignore_hidden */ false);
+
 	edges_adjust_resize_geom(view, next_edges,
 		resize_edges, geo, /* use_pending */ true);
+
+	/*
+	 * Record effective geometry after snapping in case the client opts to
+	 * ignore or modify the configured geometry
+	 */
+	snap_constraints_set(view, direction, *geo);
 }
 
 void
-snap_shrink_to_next_edge(struct view *view, enum view_edge direction,
-		struct wlr_box *geo)
+snap_shrink_to_next_edge(struct view *view,
+		enum view_edge direction, struct wlr_box *geo)
 {
 	assert(view);
 	assert(!view->shaded);
@@ -252,15 +259,23 @@ snap_shrink_to_next_edge(struct view *view, enum view_edge direction,
 	struct border next_edges;
 	edges_initialize(&next_edges);
 
+	/* Use a constrained, effective geometry for snapping if appropriate */
+	struct wlr_box origin = snap_constraints_effective(view, direction);
+
 	/* Snap to output edges if the moving edge started off-screen */
-	edges_find_outputs(&next_edges, view, *geo,
-		view->output, check_edge, /* use_pending */ true);
+	edges_find_outputs(&next_edges, view,
+		origin, *geo, view->output, check_edge);
 
 	/* Limit motion to any intervening edge of ther views on this output */
-	edges_find_neighbors(&next_edges,
-		view, *geo, view->output, check_edge,
-		/* use_pending */ true, /* ignore_hidden */ false);
+	edges_find_neighbors(&next_edges, view, origin, *geo,
+		view->output, check_edge, /* ignore_hidden */ false);
 
 	edges_adjust_resize_geom(view, next_edges,
 		resize_edges, geo, /* use_pending */ true);
+
+	/*
+	 * Record effective geometry after snapping in case the client opts to
+	 * ignore or modify the configured geometry
+	 */
+	snap_constraints_set(view, direction, *geo);
 }

src/view.c

@@ -13,6 +13,7 @@
 #include "placement.h"
 #include "regions.h"
 #include "resize_indicator.h"
+#include "snap-constraints.h"
 #include "snap.h"
 #include "ssd.h"
 #include "view.h"
@@ -2203,6 +2204,8 @@ view_destroy(struct view *view)
 	assert(view);
 	struct server *server = view->server;
 
+	snap_constraints_invalidate(view);
+
 	if (view->mappable.connected) {
 		mappable_disconnect(&view->mappable);
 	}

src/xdg.c

@@ -8,6 +8,7 @@
 #include "decorations.h"
 #include "labwc.h"
 #include "node.h"
+#include "snap-constraints.h"
 #include "view.h"
 #include "view-impl-common.h"
 #include "window-rules.h"
@@ -141,6 +142,7 @@ handle_commit(struct wl_listener *listener, void *data)
 		 * actual view.
 		 */
 		if (!view->pending_configure_serial) {
+			snap_constraints_update(view);
 			view->pending = view->current;
 
 			/*
@@ -188,6 +190,7 @@ handle_configure_timeout(void *data)
 		view->current.width, view->current.height);
 
 	/* Re-sync pending view with current state */
+	snap_constraints_update(view);
 	view->pending = view->current;
 
 	return 0; /* ignored per wl_event_loop docs */