Commitd617fc5eRecorded19 Jul 2026Repositorylantern
lantern: deliver Tab/Shift+Tab to the web content instead of GTK focus-nav
Message
GTK4 reserves Tab (focus-next) and Shift+Tab / ISOLeftTab (focus-prev) for widget focus navigation and consumes them BEFORE the embedded WebKitWebView's web content ever receives a keydown. A keyboard-first web app (Slate) therefore never sees Tab, and GTK moves focus right out of the view — e.g. Shift+Tab in a Slate terminal made the whole pane lose focus until you clicked back in. No page-side JS can fix this: the event does not reach the page at all (verified live — the page's keydown listener fires in Chrome but never in Lantern).
Fix: install a CAPTURE-phase GtkEventControllerKey on the window. on-key-pressed intercepts GDKKEYTab / GDKKEYISOLeftTab, forwards a synthetic keydown into the page (dispatched on the focused element / the app .frame, so the app's own keymap routes it — terminal backtab e[Z, editor indent, ...), and returns TRUE to consume the GTK event so focus stays in the view. The keyval alone distinguishes plain Tab from Shift+Tab (ISOLeftTab), so no GdkModifierType decode is needed. Every other key returns FALSE and flows to WebKit unchanged; on any error the guard returns FALSE, falling back to GTK's default rather than swallowing a keystroke.
v1 single-window: the callback reads main-view, set in create-window (matches the existing single-window callback shape). Confirmed working end-to-end in a Slate terminal (Claude) in a native Lantern window.
Changed
lantern/src/lantern/backend/gtk.sgl | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 66 insertions(+)Diff
lantern/src/lantern/backend/gtk.sglmodified
@@ -46,6 +46,15 @@
46
;; GTK_STYLE_PROVIDER_PRIORITY_APPLICATION - app CSS beats the theme's window bg. 47
(define STYLE-PRIORITY-APPLICATION 600) 48
+49
;; GtkPropagationPhase: CAPTURE = 1 (top-down, before the target + before GTK's+50
;; own Tab focus-navigation default fires).+51
(define GTK-PHASE-CAPTURE 1)+52
;; GDK keyvals for the focus-navigation keys GTK4 reserves. Shift+Tab arrives+53
;; as ISO_Left_Tab (a distinct keyval), so the keyval alone tells plain Tab+54
;; from Shift+Tab — no GdkModifierType decode needed.+55
(define GDK-KEY-Tab #xff09)+56
(define GDK-KEY-ISO-Left-Tab #xfe20)+57
58
;; ---- lazily-bound FFI (all #f until ensure!) ------------------------- 59
60
(define *ready* #f)@@ -67,6 +76,10 @@
76
(define gtk-css-provider-load-from-string #f) 77
(define gtk-style-context-add-provider-for-display #f) 78
(define gdk-display-get-default #f)+79
;; Key event controller (to intercept Tab/Shift+Tab before GTK's focus-nav).+80
(define gtk-event-controller-key-new #f)+81
(define gtk-event-controller-set-propagation-phase #f)+82
(define gtk-widget-add-controller #f) 83
84
(define webkit-web-context-get-default #f) 85
(define webkit-web-context-get-security-manager #f)@@ -107,6 +120,7 @@
120
(define message-cb #f) 121
(define policy-cb #f) 122
(define close-cb #f)+123
(define keypress-cb #f) 124
125
;; ---- per-process backend state --------------------------------------- 126
;;@@ -124,6 +138,9 @@
138
;; The display-wide CSS provider that clears the window background for 139
;; transparency. Installed once, rooted here for the process lifetime. 140
(define *transparency-provider* #f)+141
;; The single window's web view, so the capture-phase key handler can forward+142
;; Tab/Shift+Tab into the page (v1 is single-window; see the callback note).+143
(define *main-view* #f) 144
145
(define (path-of request) 146
(let ((p (webkit-uri-scheme-request-get-path request)))@@ -200,6 +217,39 @@
217
(set! *quit-requested* #t) 218
#f))) 219
+220
;; key-pressed on a CAPTURE-phase controller on the window (5-arg:+221
;; controller keyval keycode state user-data -> gboolean).+222
;;+223
;; GTK4 reserves Tab (focus-next) and Shift+Tab / ISO_Left_Tab (focus-prev)+224
;; for widget focus navigation and consumes them BEFORE the WebKitWebView's+225
;; web content ever sees a keydown. A keyboard-first web app (Slate) therefore+226
;; never receives Tab, and GTK moves focus right out of the view (the terminal+227
;; "loses focus" on Shift+Tab until you click back in). No page-side JS can fix+228
;; this — the event doesn't reach the page. So intercept the two focus-nav keys+229
;; HERE, forward a synthetic keydown into the page (Slate's own keymap then+230
;; routes it — terminal backtab \e[Z, editor indent, etc.), and return #t to+231
;; consume the GTK event so focus stays in the view. Every other key returns #f+232
;; and flows to WebKit normally. The keyval distinguishes plain Tab from+233
;; Shift+Tab (ISO_Left_Tab), so no modifier decode is needed. On any error the+234
;; guard returns #f — i.e. fall back to GTK's default (focus moves), never a+235
;; swallowed keystroke.+236
(define (on-key-pressed controller keyval keycode state user-data)+237
(guarded "key-pressed" #f+238
(lambda ()+239
(if (and *main-view*+240
(or (= keyval GDK-KEY-Tab) (= keyval GDK-KEY-ISO-Left-Tab)))+241
(let ((shift (if (= keyval GDK-KEY-ISO-Left-Tab) "true" "false")))+242
(webkit-web-view-eval-js *main-view*+243
(string-append+244
"(function(){var f=document.querySelector('.frame');if(!f)return;"+245
"var a=document.activeElement;"+246
"var t=(a&&f.contains(a))?a:f;"+247
"t.dispatchEvent(new KeyboardEvent('keydown',{key:'Tab',code:'Tab',"+248
"keyCode:9,which:9,shiftKey:" shift ",bubbles:true,cancelable:true}));})();")+249
-1 NULL NULL NULL NULL NULL)+250
#t)+251
#f))))+252
253
;; GdkRGBA = 4 floats in GTK4. set_background_color copies, so free at once. 254
(define (set-view-background view r g b a) 255
(let ((rgba (dict->c-struct (dict red: r green: g blue: b alpha: a) gdk-rgba-layout)))@@ -257,6 +307,10 @@
307
(set! gtk-style-context-add-provider-for-display 308
(c-function libgtk "gtk_style_context_add_provider_for_display" (list ffi/pointer ffi/pointer ffi/uint) ffi/void)) 309
(set! gdk-display-get-default (c-function libgtk "gdk_display_get_default" (list) ffi/pointer))+310
(set! gtk-event-controller-key-new (c-function libgtk "gtk_event_controller_key_new" (list) ffi/pointer))+311
(set! gtk-event-controller-set-propagation-phase+312
(c-function libgtk "gtk_event_controller_set_propagation_phase" (list ffi/pointer ffi/int) ffi/void))+313
(set! gtk-widget-add-controller (c-function libgtk "gtk_widget_add_controller" (list ffi/pointer ffi/pointer) ffi/void)) 314
315
(set! webkit-web-context-get-default (c-function libwebkit "webkit_web_context_get_default" (list) ffi/pointer)) 316
(set! webkit-web-context-get-security-manager@@ -330,6 +384,8 @@
384
(set! message-cb (c-callback (list ffi/pointer ffi/pointer ffi/pointer) ffi/void on-script-message)) 385
(set! policy-cb (c-callback (list ffi/pointer ffi/pointer ffi/int ffi/pointer) ffi/bool on-decide-policy)) 386
(set! close-cb (c-callback (list ffi/pointer ffi/pointer) ffi/bool on-close-request))+387
;; GtkEventControllerKey::key-pressed = (controller keyval keycode state user-data) -> gboolean+388
(set! keypress-cb (c-callback (list ffi/pointer ffi/uint ffi/uint ffi/uint ffi/pointer) ffi/bool on-key-pressed)) 389
390
(set! *ready* #t)))) 391
@@ -371,6 +427,16 @@
427
;; Origin gating: every navigation attempt passes through decide-policy. 428
(signal-connect! view "decide-policy" policy-cb) 429
(signal-connect! win "close-request" close-cb)+430
;; Deliver Tab/Shift+Tab to the web content instead of letting GTK4+431
;; consume them for widget focus navigation (which yanks focus out of+432
;; the view). A capture-phase key controller on the window fires before+433
;; GTK's focus-nav; on-key-pressed forwards those keys into the page and+434
;; consumes the GTK event. (v1 single-window: the callback reads *main-view*.)+435
(set! *main-view* view)+436
(let ((kc (gobject-borrow (gtk-event-controller-key-new))))+437
(gtk-event-controller-set-propagation-phase kc GTK-PHASE-CAPTURE)+438
(signal-connect! kc "key-pressed" keypress-cb)+439
(gtk-widget-add-controller win kc)) 440
(dict win: win view: view ucm: ucm))) 441
442
(define (load-uri window uri)