Fix white flash during reconnection in dark mode#1155
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Fix white flash during reconnection in dark mode#1155
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WKWebView.underPageBackgroundColor (iOS 15+) controls the WebKit rendering subprocess background, separate from the UIKit layer. Without setting it, it defaults to white even when backgroundColor is .clear and isOpaque is false — causing a visible white flash during reconnection when the DOM body is cleared and the new URL starts loading. Setting underPageBackgroundColor = .systemBackground makes this adaptive, showing dark in dark mode and white in light mode. Fixes #1153 Signed-off-by: Tim Mueller-Seydlitz <timbms@gmail.com>
WKWebView renders in a separate GPU process, so setting isOpaque=false and backgroundColor=.clear on the UIKit layer does not make the WebKit canvas itself transparent. The canvas still flashes white during navigation before content is painted. The fix adds a systemBackground-colored overlay view that covers the webview during page transitions. It is shown at the start of clearExistingPage() and didStartProvisionalNavigation, then faded out in didFinish and didFail. This hides the white flash in all cases: after body removal, during forced reloads, and on reconnection. In dark mode the overlay is near-black (matching systemBackground), replacing the white flash with a colour-appropriate transition. Fixes #1153 Signed-off-by: Tim Mueller-Seydlitz <timbms@gmail.com>
Two bugs prevented the overlay from working: 1. attachWebViewToLayout used view.addSubview() which appends the webview at the top of the z-order. On reconnection when a new webview instance is attached, it landed above the overlay, making it invisible. Fixed by using insertSubview(_:at:0) to always place webviews at the bottom of the stack. 2. The overlay started at alpha=0, leaving the initial blank webview unprotected before the first didStartProvisionalNavigation fires. Fixed by starting at alpha=1 so coverage is immediate from viewDidLoad. Signed-off-by: Tim Mueller-Seydlitz <timbms@gmail.com>
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Summary
WKWebView.underPageBackgroundColorcontrols the WebKit rendering subprocess background, which is separate from the UIKit compositing layerbackgroundColor = .clearandisOpaque = false— causing the white flash reported in Reconnection produces a “white page” that flashes briefly and blinds me frequently #1153.systemBackgroundmakes it adaptive (dark in dark mode, white in light mode)Test plan
Fixes #1153