QLineEdit: implement quick text selection by mouse

This is a standard feature in GtkEntry widgets or HTML
<input type="text"> elements. During a normal text selection by mouse
(LeftButton press + mouse move event), it's now possible to quickly
select all the text from the start of the selection to the end
of the line edit by moving the mouse cursor down.
By moving it up instead, all the text up to the start of the line edit
gets selected. If the layout direction is right-to-left, the semantic of
the mouse movement is inverted.

This feature is only enabled if the y() of the mouse move event is
bigger than a fixed threshold, to avoid unexpected selections in the
normal case. This threshold is set by the QPlatformTheme and a value
smaller than zero disables this feature.

The threshold is updated whenever the style or the screen changes.

[ChangeLog][QtWidgets][QLineEdit] Implemented quick text selection by
mouse in QLineEdit.

Change-Id: I4de33c2d11c033ec295de2b2ea81adf786324f4b
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/gui/kernel/qplatformintegration.cpp b/src/gui/kernel/qplatformintegration.cpp
index 151151de23..448d670209 100644
--- a/src/gui/kernel/qplatformintegration.cpp
+++ b/src/gui/kernel/qplatformintegration.cpp
@@ -418,6 +418,8 @@ QVariant QPlatformIntegration::styleHint(StyleHint hint) const
         return QPlatformTheme::defaultThemeHint(QPlatformTheme::UiEffects);
     case WheelScrollLines:
         return QPlatformTheme::defaultThemeHint(QPlatformTheme::WheelScrollLines);
+    case MouseQuickSelectionThreshold:
+        return QPlatformTheme::defaultThemeHint(QPlatformTheme::MouseQuickSelectionThreshold);
     }
 
     return 0;