tiling drag: only start when there are drop targets (#5213)

This prevents potentially confusing drag & drop on fullscreen containers and
only-containers on workspaces.

fixes https://github.com/i3/i3/issues/5184

4 files changed, 76 insertions, 18 deletions

diff --git a/include/tiling_drag.h b/include/tiling_drag.h
index d78744c9..3091b734 100644
--- a/include/tiling_drag.h
+++ b/include/tiling_drag.h
@@ -9,6 +9,8 @@
  */
 #pragma once
 
+#include "all.h"
+
 /**
  * Tiling drag initiation modes.
  */
@@ -20,6 +22,13 @@ typedef enum {
 } tiling_drag_t;
 
 /**
+ * Returns whether there currently are any drop targets.
+ * Used to only initiate a drag when there is something to drop onto.
+ *
+ */
+bool has_drop_targets(void);
+
+/**
  * Initiates a mouse drag operation on a tiled window.
  *
  */
diff --git a/release-notes/changes/2-tiling-drag-targets b/release-notes/changes/2-tiling-drag-targets
new file mode 100644
index 00000000..9981a199
--- /dev/null
+++ b/release-notes/changes/2-tiling-drag-targets
@@ -0,0 +1 @@
+tiling drag: only initiate when there are drop targets
diff --git a/src/click.c b/src/click.c
index 6f1cb5c2..e262516f 100644
--- a/src/click.c
+++ b/src/click.c
@@ -230,7 +230,8 @@ static void route_click(Con *con, xcb_button_press_event_t *event, const bool mo
     /* 2: floating modifier pressed, initiate a drag */
     if (mod_pressed && is_left_click && !floatingcon &&
         (config.tiling_drag == TILING_DRAG_MODIFIER ||
-         config.tiling_drag == TILING_DRAG_MODIFIER_OR_TITLEBAR)) {
+         config.tiling_drag == TILING_DRAG_MODIFIER_OR_TITLEBAR) &&
+        has_drop_targets()) {
         const bool use_threshold = !mod_pressed;
         tiling_drag(con, event, use_threshold);
         allow_replay_pointer(event->time);
@@ -311,7 +312,8 @@ static void route_click(Con *con, xcb_button_press_event_t *event, const bool mo
     if (is_left_click &&
         ((config.tiling_drag == TILING_DRAG_TITLEBAR && dest == CLICK_DECORATION) ||
          (config.tiling_drag == TILING_DRAG_MODIFIER_OR_TITLEBAR &&
-          (mod_pressed || dest == CLICK_DECORATION)))) {
+          (mod_pressed || dest == CLICK_DECORATION))) &&
+        has_drop_targets()) {
         allow_replay_pointer(event->time);
         const bool use_threshold = !mod_pressed;
         tiling_drag(con, event, use_threshold);
diff --git a/src/tiling_drag.c b/src/tiling_drag.c
index 94908872..a35a66e6 100644
--- a/src/tiling_drag.c
+++ b/src/tiling_drag.c
@@ -27,6 +27,62 @@ static Rect con_rect_plus_deco_height(Con *con) {
     return rect;
 }
 
+static bool is_tiling_drop_target(Con *con) {
+    if (!con_has_managed_window(con) ||
+        con_is_floating(con) ||
+        con_is_hidden(con)) {
+        return false;
+    }
+    Con *ws = con_get_workspace(con);
+    if (con_is_internal(ws)) {
+        /* Skip containers on i3-internal containers like the scratchpad, which are
+           technically visible on their pseudo-output. */
+        return false;
+    }
+    if (!workspace_is_visible(ws)) {
+        return false;
+    }
+    Con *fs = con_get_fullscreen_covering_ws(ws);
+    if (fs != NULL && fs != con) {
+        /* Workspace is visible, but con is not visible because some other
+           container is in fullscreen. */
+        return false;
+    }
+    return true;
+}
+
+/*
+ * Returns whether there currently are any drop targets.
+ * Used to only initiate a drag when there is something to drop onto.
+ *
+ */
+bool has_drop_targets(void) {
+    int drop_targets = 0;
+    Con *con;
+    TAILQ_FOREACH (con, &all_cons, all_cons) {
+        if (!is_tiling_drop_target(con)) {
+            continue;
+        }
+        drop_targets++;
+    }
+
+    /* In addition to tiling containers themselves, an visible but empty
+     * workspace (in a multi-monitor scenario) also is a drop target. */
+    Con *output;
+    TAILQ_FOREACH (output, &(croot->focus_head), focused) {
+        if (con_is_internal(output)) {
+            continue;
+        }
+        Con *visible_ws = NULL;
+        GREP_FIRST(visible_ws, output_get_content(output), workspace_is_visible(child));
+        if (visible_ws != NULL && con_num_children(visible_ws) == 0) {
+            drop_targets++;
+        }
+    }
+
+    return drop_targets > 1;
+}
+
 /*
  * Return an appropriate target at given coordinates.
  *
@@ -35,23 +91,13 @@ static Con *find_drop_target(uint32_t x, uint32_t y) {
     Con *con;
     TAILQ_FOREACH (con, &all_cons, all_cons) {
         Rect rect = con_rect_plus_deco_height(con);
-
-        if (rect_contains(rect, x, y) &&
-            con_has_managed_window(con) &&
-            !con_is_floating(con) &&
-            !con_is_hidden(con)) {
-            Con *ws = con_get_workspace(con);
-            if (strcmp(ws->name, "__i3_scratch") == 0) {
-                /* Skip containers on the scratchpad, which are technically
-                   visible on their pseudo-output. */
-                continue;
-            }
-            if (!workspace_is_visible(ws)) {
-                continue;
-            }
-            Con *fs = con_get_fullscreen_covering_ws(ws);
-            return fs ? fs : con;
+        if (!rect_contains(rect, x, y) ||
+            !is_tiling_drop_target(con)) {
+            continue;
         }
+        Con *ws = con_get_workspace(con);
+        Con *fs = con_get_fullscreen_covering_ws(ws);
+        return fs ? fs : con;
     }
 
     /* Couldn't find leaf container, get a workspace. */