From b869db728b6bbefa098ac987a796b2b0b836c7cd Mon Sep 17 00:00:00 2001
From: Emil Ernerfeldt <emilernerfeldt@gmail.com>
Date: Sun, 17 Jan 2021 12:22:19 +0100
Subject: [PATCH] [egui_web] Call prevent_default for arrow keys when entering
 text

Closes https://github.com/emilk/egui/issues/58
---
 egui_web/CHANGELOG.md   |  4 ++++
 egui_web/src/backend.rs |  4 ++++
 egui_web/src/lib.rs     | 47 ++++++++++++++++++++++++-----------------
 3 files changed, 36 insertions(+), 19 deletions(-)

diff --git a/egui_web/CHANGELOG.md b/egui_web/CHANGELOG.md
index 93c99f9d..b66ee085 100644
--- a/egui_web/CHANGELOG.md
+++ b/egui_web/CHANGELOG.md
@@ -15,6 +15,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 * Slightly improved alpha-blending (work-around for non-existing linear-space blending).
 
+### Fixed ⭐
+
+* Call prevent_default for arrow keys when entering text
+
 
 ## 0.7.0 - 2021-01-04
 
diff --git a/egui_web/src/backend.rs b/egui_web/src/backend.rs
index 51de6c82..90e9842b 100644
--- a/egui_web/src/backend.rs
+++ b/egui_web/src/backend.rs
@@ -154,6 +154,10 @@ impl AppRunner {
         })
     }
 
+    pub fn egui_ctx(&self) -> &egui::CtxRef {
+        &self.web_backend.ctx
+    }
+
     pub fn auto_save(&mut self) {
         let now = now_sec();
         let time_since_last_save = now - self.last_save_time;
diff --git a/egui_web/src/lib.rs b/egui_web/src/lib.rs
index a5edf9e2..a2c57c15 100644
--- a/egui_web/src/lib.rs
+++ b/egui_web/src/lib.rs
@@ -452,25 +452,34 @@ fn install_document_events(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
             }
             runner_lock.needs_repaint.set_true();
 
-            // So, shall we call prevent_default?
-            // YES:
-            // * Tab  (move to next text field)
-            //
-            // SOMETIMES:
-            // * Backspace - when entering text we don't want to go back one page.
-            //
-            // NO:
-            // * F5 / cmd-R (refresh)
-            // * cmd-shift-C (debug tools)
-            // * ...
-            //
-            // NOTE: if we call prevent_default for cmd-c/v/x, we will prevent copy/paste/cut events.
-            // Let's do things manually for now:
-            if matches!(
-                event.key().as_str(),
-                "Backspace"  // so we don't go back to previous page when deleting text
-                | "Tab" // so that e.g. tab doesn't move focus to url bar
-            ) {
+            let egui_wants_keyboard = runner_lock.egui_ctx().wants_keyboard_input();
+
+            let prevent_default = if matches!(event.key().as_str(), "Tab") {
+                // Always prevent moving cursor to url bar.
+                // Egui wants to use tab to move to the next text field.
+                true
+            } else if egui_wants_keyboard {
+                matches!(
+                    event.key().as_str(),
+                    "Backspace" // so we don't go back to previous page when deleting text
+                | "ArrowDown" | "ArrowLeft" | "ArrowRight" | "ArrowUp" // cmd-left is "back" on Mac (https://github.com/emilk/egui/issues/58)
+                )
+            } else {
+                // We never want to prevent:
+                // * F5 / cmd-R (refresh)
+                // * cmd-shift-C (debug tools)
+                // * cmd/ctrl-c/v/x (or we stop copy/past/cut events)
+                false
+            };
+
+            // console_log(format!(
+            //     "On key-down {:?}, egui_wants_keyboard: {}, prevent_default: {}",
+            //     event.key().as_str(),
+            //     egui_wants_keyboard,
+            //     prevent_default
+            // ));
+
+            if prevent_default {
                 event.prevent_default();
             }
         }) as Box<dyn FnMut(_)>);