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fix: keep back arrow visible until the exiting screen finishes leaving

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canPop() read the globally-current back-stack entry via
currentBackStackEntryAsState(). A pop commits the instant it is accepted
— most visibly during a predictive back-swipe, whose exit animation is
long and finger-driven — so controller.currentBackStackEntry flips to the
destination while the screen being dismissed is still on screen, sliding
out and still composing its top bar. Evaluating canPop against the
incoming destination there dropped the back arrow (and re-showed the
bottom bar) before the outgoing screen had finished leaving, which is
exactly the flicker seen when back-swiping to Home or a bottom-nav root.

Evaluate poppability against the screen's own NavBackStackEntry — the one
the NavHost provides to each destination via LocalViewModelStoreOwner —
which is intrinsic to that screen and never changes for the life of its
composition. The arrow now stays put until the screen itself is gone.
Callers outside a NavHost destination (shell chrome, drawer) see the
account-scoped owner instead of an entry and fall back to the previous
globally-current behavior.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011matx1mGJ6ZyS5dS77EQUo
This commit is contained in:
Claude
2026-07-28 20:03:43 +00:00
parent f3db2c73dc
commit 88ee46df05
1 changed files with 30 additions and 13 deletions
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amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/navs/Nav.kt
+30 -13
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@@ -28,9 +28,10 @@ import androidx.compose.runtime.Stable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.lifecycle.viewmodel.compose.LocalViewModelStoreOwner
import androidx.navigation.NavBackStackEntry
import androidx.navigation.NavGraph.Companion.findStartDestination
import androidx.navigation.NavHostController
import androidx.navigation.compose.currentBackStackEntryAsState
import com.vitorpamplona.amethyst.ui.navigation.BOTTOM_NAV_ROOT_KEY
import com.vitorpamplona.amethyst.ui.navigation.isBottomNavRoot
import com.vitorpamplona.amethyst.ui.navigation.routes.Route
@@ -115,19 +116,35 @@ class Nav(
@Composable
override fun canPop(): Boolean {
// Observe the current entry as State so consumers recompose when the
// back stack settles after a navigation or back-swipe transition.
// A non-reactive read would leave a stale value behind: e.g. on
// back-swipe to Home, previousBackStackEntry is still the popping
// entry until the gesture finishes, and nothing would re-evaluate
// canPop afterwards.
val current by controller.currentBackStackEntryAsState()
val entry = current ?: return false
// Decide the back arrow / bottom-bar visibility from THIS screen's own
// back-stack entry — the one the NavHost hands to each destination
// through LocalViewModelStoreOwner — instead of the globally-current
// entry.
//
// A pop commits the moment it is accepted (most visibly during a
// predictive back-swipe, whose exit animation is long and finger-driven):
// controller.currentBackStackEntry flips to the destination while the
// screen being dismissed is still on screen, sliding out and still
// composing its top bar. Reading the global entry there re-evaluated
// canPop against the incoming destination and dropped the arrow before
// the outgoing screen had finished leaving. An entry is intrinsic to its
// screen and never changes for the life of that composition, so the arrow
// now stays put until the screen itself is gone.
//
// Outside a NavHost destination (shell chrome, drawer) the current owner
// is the account-scoped ViewModelStoreOwner, not an entry; fall back to
// the globally-current entry so those callers keep their prior behavior.
val entry =
(LocalViewModelStoreOwner.current as? NavBackStackEntry)
?: controller.currentBackStackEntry
?: return false
// Hidden on tab roots (reached via the bottom nav) and on Home (the
// graph's start destination): nothing sits below either that a back
// arrow could return to. Every other entry is a push on top of Home,
// so it can always pop.
if (entry.isBottomNavRoot()) return false
// Home is the graph's start destination and nothing can sit below
// it, so a back arrow there is never meaningful.
if (entry.destination.id == controller.graph.findStartDestination().id) return false
return controller.previousBackStackEntry != null
return entry.destination.id != controller.graph.findStartDestination().id
}
override fun popBack() {