[client] keybind: display help overlay while ScrollLock is held

This overlay will show the list of keybindings.

client/include/app.h

@@ -110,5 +110,9 @@ void app_releaseKeybind(KeybindHandle * handle);
  */
 void app_releaseAllKeybinds(void);
 
+/**
+ * Changes whether the help message is displayed or not.
+ */
+void app_showHelp(bool show);
 
 #endif

client/src/app.c

@@ -189,6 +189,7 @@ void app_handleKeyPress(int sc)
   {
     g_state.escapeActive = true;
     g_state.escapeAction = -1;
+    app_showHelp(true);
     return;
   }
 
@@ -240,7 +241,10 @@ void app_handleKeyRelease(int sc)
     }
 
     if (sc == g_params.escapeKey)
+    {
       g_state.escapeActive = false;
+      app_showHelp(false);
+    }
   }
 
   if (!core_inputEnabled())
@@ -496,3 +500,55 @@ void app_releaseAllKeybinds(void)
       g_state.bindings[i] = NULL;
     }
 }
+
+static char * build_help_str()
+{
+  size_t size   = 50;
+  size_t offset = 0;
+  char * buffer = malloc(size);
+
+  if (!buffer)
+    return NULL;
+
+  const char * escapeName = xfree86_to_display[g_params.escapeKey];
+
+  offset += snprintf(buffer, size, "%s %-10s Toggle capture mode\n", escapeName, "");
+  if (offset >= size)
+  {
+    DEBUG_ERROR("Help string somehow overflowed. This should be impossible.");
+    return NULL;
+  }
+
+  for (int i = 0; i < KEY_MAX; ++i)
+  {
+    if (g_state.keyDescription[i])
+    {
+      const char * keyName = xfree86_to_display[i];
+      const char * desc    = g_state.keyDescription[i];
+      int needed = snprintf(buffer + offset, size - offset, "%s+%-10s %s\n", escapeName, keyName, desc);
+      if (offset + needed < size)
+        offset += needed;
+      else
+      {
+        size = size * 2 + needed;
+        void * new = realloc(buffer, size);
+        if (!new) {
+          free(buffer);
+          DEBUG_ERROR("Out of memory when constructing help text");
+          return NULL;
+        }
+        buffer = new;
+        offset += snprintf(buffer + offset, size - offset, "%s+%-10s %s\n", escapeName, keyName, desc);
+      }
+    }
+  }
+
+  return buffer;
+}
+
+void app_showHelp(bool show)
+{
+  char * help = show ? build_help_str() : NULL;
+  g_state.lgr->on_help(g_state.lgrData, help);
+  free(help);
+}