Add a setting for window transparency

On some setups, it looks like setting WA_TranslucentBackground causes
worse performance. Additionally, it also breaks dmenu window embedding
with its `-w` flag. This adds a setting to make it optional.

I originally tried to set this in a config change handler, but even with
a subsequent .hide()/.show(), for some odd reason that causes
rendering/repainting artifacts, possibly due to a Qt bug.

Follow-up to #5546
Closes #5805

doc/help/settings.asciidoc

@@ -327,6 +327,7 @@
 |<<url.yank_ignored_parameters,url.yank_ignored_parameters>>|URL parameters to strip with `:yank url`.
 |<<window.hide_decoration,window.hide_decoration>>|Hide the window decoration.
 |<<window.title_format,window.title_format>>|Format to use for the window title. The same placeholders like for
+|<<window.transparent,window.transparent>>|Set the main window background to transparent.
 |<<zoom.default,zoom.default>>|Default zoom level.
 |<<zoom.levels,zoom.levels>>|Available zoom levels.
 |<<zoom.mouse_divider,zoom.mouse_divider>>|Number of zoom increments to divide the mouse wheel movements to.
@@ -4206,6 +4207,22 @@ Type: <<types,FormatString>>
 
 Default: +pass:[{perc}{current_title}{title_sep}qutebrowser]+
 
+[[window.transparent]]
+=== window.transparent
+Set the main window background to transparent.
+
+This allows having a transparent tab- or statusbar (might require a compositor such
+as picom). However, it breaks some functionality such as dmenu embedding via its
+`-w` option. On some systems, it was additionally reported that main window
+transparency negatively affects performance.
+
+Note this setting only affects windows opened after setting it.
+
+
+Type: <<types,Bool>>
+
+Default: +pass:[false]+
+
 [[zoom.default]]
 === zoom.default
 Default zoom level.

qutebrowser/config/configdata.yml

@@ -2074,6 +2074,19 @@ window.title_format:
     Format to use for the window title. The same placeholders like for
     `tabs.title.format` are defined.
 
+window.transparent:
+  type: Bool
+  default: false
+  desc: |
+    Set the main window background to transparent.
+
+    This allows having a transparent tab- or statusbar (might require a compositor such
+    as picom). However, it breaks some functionality such as dmenu embedding via its
+    `-w` option. On some systems, it was additionally reported that main window
+    transparency negatively affects performance.
+
+    Note this setting only affects windows opened after setting it.
+
 ## zoom
 
 zoom.default:

qutebrowser/mainwindow/mainwindow.py

@@ -203,8 +203,10 @@ class MainWindow(QWidget):
         from qutebrowser.mainwindow.statusbar import bar
 
         self.setAttribute(Qt.WA_DeleteOnClose)
-        self.setAttribute(Qt.WA_TranslucentBackground)
-        self.palette().setColor(QPalette.Window, Qt.transparent)
+        if config.val.window.transparent:
+            self.setAttribute(Qt.WA_TranslucentBackground)
+            self.palette().setColor(QPalette.Window, Qt.transparent)
+
         self._overlays: MutableSequence[_OverlayInfoType] = []
         self.win_id = next(win_id_gen)
         self.registry = objreg.ObjectRegistry()