How can I remove the border on a GTK tooltip?

Question

I am trying to precisely control the content of a GTK tooltip, but whatever I do, there is always a border remaining.

My real content is much more advanced, but the problem is 100% reproduced with just a gtk_label. The gtk container whose class is tooltip and which contains my custom widget has a weird border/padding behavior: if I add border, only the 6 outer pixels are visible, and if the border drops below 6px, a weird extra padding using the background-color takes over, and the effective border never gets below these 6px:

Border around tooltip

Here is the minimal source code to reproduce with border: 0;:

// Compile with:
// gcc tooltip.c -o tooltip `pkg-config --cflags --libs gtk+-3.0`
#include <gtk/gtk.h>
static gboolean on_query_tooltip(GtkWidget *, gint, gint, gboolean, GtkTooltip *tooltip, gpointer) {
 GtkWidget *label = gtk_label_new(NULL);
 gtk_label_set_text(GTK_LABEL(label), "Single content.");
 gtk_widget_show_all(label);
 gtk_tooltip_set_custom(tooltip, label);
 return TRUE;
}
int main(int argc, char **argv) {
 gtk_init(&argc, &argv);
 const gchar *css =
 "tooltip {"
 " background-color: #f00;"
 " padding: 0;"
 " margin: 0;"
 " border: 0;"
 " border-radius: 0;"
 "}"
 "tooltip > box {"
 " background-color: #00f;"
 " padding: 0;"
 " margin: 0;"
 " border: 0;"
 "}"
 "tooltip > box > label {"
 " background-color: #0f0;"
 " color: #000;"
 " padding: 0;"
 " margin: 0;"
 " border: 0;"
 " text-shadow: none;"
 "}";
 GtkCssProvider *prov = gtk_css_provider_new();
 GError *err = NULL;
 gtk_css_provider_load_from_data(prov, css, -1, &err);
 if (err) { g_printerr("CSS error: %s\n", err->message); g_error_free(err); }
 gtk_style_context_add_provider_for_screen(
 gdk_screen_get_default(), GTK_STYLE_PROVIDER(prov),
 GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 g_object_unref(prov);
 GtkWidget *win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
 gtk_window_set_default_size(GTK_WINDOW(win), 200, 100);
 g_signal_connect(win, "destroy", G_CALLBACK(gtk_main_quit), NULL);
 GtkWidget *label = gtk_label_new("hover me");
 gtk_widget_set_halign(label, GTK_ALIGN_CENTER);
 gtk_widget_set_valign(label, GTK_ALIGN_CENTER);
 gtk_widget_set_has_tooltip(label, TRUE);
 g_signal_connect(label, "query-tooltip", G_CALLBACK(on_query_tooltip), NULL);
 gtk_container_add(GTK_CONTAINER(win), label);
 gtk_widget_show_all(win);
 gtk_main();
 return 0;
}

I suspect that this 6px border was hardcoded somewhere to make the default border-radius: 6px work, but I can't find any reference to this.

Answer 1

This 6px border is hard coded, it can't be modified by a GTK program. The solution is to use my own tooltip window. Here is a minimal example for anyone having the same issue.

#include <gtk/gtk.h>
static gboolean
on_query_tooltip(GtkWidget *widget, gint x, gint y, gboolean keyboard_mode,
 GtkTooltip *tooltip, gpointer user_data)
{
 // We are using our own tooltip window; do NOT use gtk_tooltip_set_custom() here.
 GtkWindow *tw = gtk_widget_get_tooltip_window(widget);
 if (!tw) return FALSE; // shouldn't happen once we've set a custom window
 // The window is a GtkBin -\> get its first child (our box)
 GtkWidget *box = gtk_bin_get_child(GTK_BIN(tw));
 // Clear any previous content
 if (box) {
 GList *children = gtk_container_get_children(GTK_CONTAINER(box));
 for (GList *l = children; l; l = l-\>next)
 gtk_container_remove(GTK_CONTAINER(box), GTK_WIDGET(l-\>data));
 g_list_free(children);
 }
 // Add your content
 GtkWidget *label = gtk_label_new("Single content.");
 gtk_container_add(GTK_CONTAINER(box), label);
 gtk_widget_show_all(GTK_WIDGET(box));
 return TRUE;
}
int main(int argc, char **argv)
{
 gtk_init(&argc, &argv);
 const gchar *css =
 ".tooltip {"
 " background-color: #fff;"
 "}";
 GtkCssProvider *prov = gtk_css_provider_new();
 gtk_css_provider_load_from_data(prov, css, -1, NULL);
 gtk_style_context_add_provider_for_screen(gdk_screen_get_default(),
 GTK_STYLE_PROVIDER(prov), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 g_object_unref(prov);
 GtkWidget *win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
 gtk_window_set_default_size(GTK_WINDOW(win), 200, 100);
 g_signal_connect(win, "destroy", G_CALLBACK(gtk_main_quit), NULL);
 GtkWidget *label = gtk_label_new("hover me");
 gtk_widget_set_halign(label, GTK_ALIGN_CENTER);
 gtk_widget_set_valign(label, GTK_ALIGN_CENTER);
 gtk_container_add(GTK_CONTAINER(win), label);
 // Create our own tooltip window (no 6px margins)
 GtkWidget *tipwin = gtk_window_new(GTK_WINDOW_POPUP);
 gtk_window_set_type_hint(GTK_WINDOW(tipwin), GDK_WINDOW_TYPE_HINT_TOOLTIP);
 // Give it the usual tooltip classes so CSS applies
 GtkStyleContext *ctx = gtk_widget_get_style_context(tipwin);
 gtk_style_context_add_class(ctx, "tooltip");
 GtkWidget *box = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 0);
 gtk_container_add(GTK_CONTAINER(tipwin), box);
 gtk_widget_show(box);
 // Use our window for this widget’s tooltip
 gtk_widget_set_tooltip_window(label, GTK_WINDOW(tipwin));
 gtk_widget_set_has_tooltip(label, TRUE);
 g_signal_connect(label, "query-tooltip", G_CALLBACK(on_query_tooltip), NULL);
 gtk_widget_show_all(win);
 gtk_main();
 return 0;
}