Wait for the text layer in text_selection_action_icons

The test dragged a selection after a fixed four frames, but the page's text
layer arrives from the render pool in the background; under load it was
sometimes not in yet, the drag selected nothing and the assertion failed
(3 of 20 runs of the editing binary locally). Wait until DocView reports
the page's text instead, via a new has_page_text accessor.

crates/ui-egui/src/canvas.rs

@@ -1464,6 +1464,12 @@ impl DocView {
         self.texts.get(&page).cloned()
     }
 
+    /// Whether `page`'s text layer has arrived from the renderer. Selecting text, the I-beam and
+    /// find matches on a page all wait for it (tests and automation).
+    pub fn has_page_text(&self, page: usize) -> bool {
+        self.texts.contains_key(&page)
+    }
+
     pub(crate) fn clear_selection(&mut self) {
         self.selection = None;
     }

crates/ui-egui/tests/editing.rs

@@ -3052,7 +3052,9 @@ fn tiny_image_crop_handle_drag_does_not_panic() {
 #[test]
 fn text_selection_action_icons() {
     let mut h = harness(1, |_| {});
-    h.run_steps(4);
+    // Dragging selects only once the page's text layer is in, and it comes from the render pool
+    // in the background: under load a fixed number of frames was sometimes not enough.
+    support::wait_until(&mut h, "page 1's text layer", |app| app.views[0].has_page_text(0));
     let r = h.state().views[0].page_screen_rect(0).expect("on screen");
     let p0 = egui::pos2(r.left() + 40.0 / 200.0 * r.width(), r.top() + (300.0 - 158.0) / 300.0 * r.height());
     let p1 = egui::pos2(r.left() + 100.0 / 200.0 * r.width(), r.top() + (300.0 - 158.0) / 300.0 * r.height());